What things should a human soul attach itself to?

Happy is the pure soul who does not attach himself to transient conditions and comforts, but rather seeks to attach himself to the purity, nobility and splendor of the world which endures. 
(‘Abdu’l-Baha, Baha'i Scriptures; Star of the West, vol. IV, No. 16, December 31, 1913)

Meaning of the term “soul” in various kingdoms of creation -- 'Abdu'l-Baha explains

As to the question concerning the soul, know thou, verily, that "soul" is a term applied to numerous realities, according to the exigencies of the following relations in regard to development in the world of existence:

(1) In the mineral kingdom, soul is called "latent force," silently working for the disintegration of the substance of the mineral.

(2) In the vegetable kingdom it is called “virtue augmentative," or the power of growth, which attracts and absorbs the delicate materials of inorganic substance found in the mineral kingdom of matter, and transforms them into the condition of growth. Thus the inorganic substance found in the mineral kingdom becomes growing vegetable life through the effect of the word of God. This vegetable soul, i. e. "virtue augmentative," or power of growth, is a quality which is produced by the admixture of elements, and appears in accidental organisms, of which contingency is an essential attribute.

Prayer is the “spiritual nourishment” for the human soul

It is the soul of man that has first to be fed. And this spiritual nourishment prayer can best provide. Laws and institutions, as viewed by Bahá'u'lláh, can become really effective only when our inner spiritual life has been perfected and transformed. Otherwise religion will degenerate into a mere organization, and become a dead thing. 
(From a letter dated 8 December 1935 written on behalf of Shoghi Effendi to an individual believer, published in "Bahá'í News" 102 (August 1936), p. 3; The Compilation of Compilations, Vol. II, The Importance of Prayer, Meditation and the Devotional Attitude)

The “body is dependent upon the soul”

Furthermore, many say, `We have no need of divine Manifestations; we ourselves have direct communication with God.' They do not know that the divine Manifestations are the bright rays of the Sun of Truth and a means of educating the realities of man. Therefore, he who rejects the bounty of the Sun of Truth and thinks himself not in need of it is like the one who says he is not in need of God and rejects both God and reality, in spite of the fact that all creation is receiving incessant bounty from God and is dependent on Him, as the body is dependent upon the soul. 
(Words of ‘Abdu’l-Baha, New York, November 26, 1912, Mahmud’s Diary)

“…greed and envy becloudeth the radiance of the soul even as the clouds obstruct the light of the sun.”

O children of understanding! If the eyelid, however delicate, can deprive man’s outer eye from beholding the world and all that is therein, consider then what would be wrought if the veil of covetousness were to descend upon his inner eye. Say: O people! The darkness of greed and envy becloudeth the radiance of the soul even as the clouds obstruct the light of the sun. Should anyone hearken unto this utterance with a discerning ear, he will unfurl the wings of detachment and soar effortlessly in the atmosphere of true understanding. 
(Baha’u’llah, ‘The Tabernacle of Unity, Bahá’u’lláh’s Responses to Mánikchí Sáhib and Other Writings’)

How a human soul can become nearer to God – ‘Abdu’l-Baha explains

Nearness to God is dependent upon purity of the heart and exhilaration of the spirit through the glad tidings of the Kingdom. Consider how a pure, well-polished mirror fully reflects the effulgence of the sun, no matter how distant the sun may be. As soon as the mirror is cleaned and purified, the sun will manifest itself. The more pure and sanctified the heart of man becomes, the nearer it draws to God, and the light of the Sun of Reality is revealed within it. This light sets hearts aglow with the fire of the love of God, opens in them the doors of knowledge and unseals the divine mysteries so that spiritual discoveries are made possible. All the Prophets have drawn near to God through severance. We must emulate those Holy Souls and renounce our own wishes and desires. We must purify ourselves from the mire and soil of earthly contact until our hearts become as mirrors in clearness and the light of the most great guidance reveals itself in them.

Bahá’u’lláh proclaims in the Hidden Words that God inspires His servants and is revealed through them. He says, “Thy heart is My home; sanctify it for My descent. Thy spirit is My place of revelation; cleanse it for My manifestation.” Therefore, we learn that nearness to God is possible through devotion to Him, through entrance into the Kingdom and service to humanity; it is attained by unity with mankind and through loving-kindness to all; it is dependent upon investigation of truth, acquisition of praiseworthy virtues, service in the cause of universal peace and personal sanctification. In a word, nearness to God necessitates sacrifice of self, severance and the giving up of all to Him. Nearness is likeness.

Behold how the sun shines upon all creation, but only surfaces that are pure and polished can reflect its glory and light. 
(‘Abdu’l-Baha, from a talk, 26 May 1912, New York, ‘The Promulgation of the Universal Peace’) (To see the entire talk please visit Talks of ‘Abdu’l-Baha)

“..Mental faculties .. are in truth of the inherent properties of the soul

...Now concerning mental faculties, they are in truth of the inherent properties of the soul, even as the radiation of light is the essential property of the sun. The rays of the sun are renewed but the sun itself is ever the same and unchanged. Consider how the human intellect develops and weakens, and may at times come to naught, whereas the soul changeth not. For the mind to manifest itself, the human body must be whole; and a sound mind cannot be but in a sound body, whereas the soul dependeth not upon the body. It is through the power of the soul that the mind comprehendeth, imagineth and exerteth its influence, whilst the soul is a power that is free. The mind comprehendeth the abstract by the aid of the concrete, but the soul hath limitless manifestations of its own. The mind is circumscribed, the soul limitless. It is by the aid of such senses as those of sight, hearing, taste, smell and touch, that the mind comprehendeth, whereas the soul is free from all agencies. The soul as thou observest, whether it be in sleep or waking, is in motion and ever active. Possibly it may, whilst in a dream, unravel an intricate problem, incapable of solution in the waking state. The mind, moreover, understandeth not whilst the senses have ceased to function, and in the embryonic stage and in early infancy the reasoning power is totally absent, whereas the soul is ever endowed with full strength. In short, the proofs are many that go to show that despite the loss of reason, the power of the soul would still continue to exist. 
(‘Abdu’l-Baha, ‘Tablet to August Forel’)

"That which is born of the flesh is flesh; and that which is born of the Spirit is spirit"(John 3:6) – explanation by ‘Abdu’l-Baha

Christ announced, "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit," meaning that man must be born again. As the babe is born into the light of this physical world, so must the physical and intellectual man be born into the light of the world of Divinity. 
('Abdu'l-Baha, The Promulgation of Universal Peace)

“…physical ailments, no matter how severe, cannot bring any change in the inherent condition of the soul…”

Regarding your questions concerning the condition of the soul during illness. The passages in the "Gleanings" make it quite clear that physical ailments, no matter how severe, cannot bring any change in the inherent condition of the soul, As Baha'u'llah says: 'The spirit is permanent and steadfast in its station'. The veil or hindrance that interposes between soul and body during physical disease is sickness itself. Sickness reveals a lack of balance in human organism, an absence of equilibrium in the forces essential for the normal functioning of the human body. 
(From a letter written on behalf of Shoghi Effendi to an individual believer, March 8, 1936; compilation: ‘Lights of Guidance’)

When the “seed of the spirit” starts to “grow in the human soul”

When a person becomes a Baha'i, actually what takes place is that the seed of the spirit starts to grow in the human soul. This seed must be watered by the outpourings of the Holy Spirit. These gifts of the spirit are received through prayer, meditation, study of the Holy Utterances and service to the Cause of God. The fact of the matter is that service in the Cause is like the plough which ploughs the physical soil when seeds are sown. It is necessary that the soil be ploughed up, so that it can be enriched, and thus cause a stronger growth of the seed. In exactly the same way the evolution of the spirit takes place through ploughing up the soil of the heart so that it is a constant reflection of the Holy Spirit. In this way the human spirit grows and develops by leaps and bounds.

Naturally there will be periods of distress and difficulty, and even severe tests; but if that person turns firmly toward the divine Manifestation, studies carefully His spiritual teachings and receives the blessings of the Holy Spirit, he will find that in reality these tests and difficulties have been the gifts of God to enable him to grow and develop." 
(From a letter written on behalf of the Guardian to an individual believer, October 6, 1954; compilation: Living the Life; compilation: Lights of Guidance)

Spiritual fasting assists the soul “to refresh and reinvigorate the spiritual forces latent” in it

Shoghi Effendi indicates that the fasting period, which involves complete abstention from food and drink from sunrise till sunset, is ‘...essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, therefore, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.’ 
(The Universal House of Justice, ‘Notes’ section of the ‘Kitab-i-Aqdas’)

God has “endowed every soul with the capacity to recognize the signs of God”

He[God] hath endowed every soul with the capacity to recognize the signs of God. How could He, otherwise, have fulfilled His testimony unto men, if ye be of them that ponder His Cause in their hearts. He will never deal unjustly with any one, neither will He task a soul beyond its power. He, verily, is the Compassionate, the All-Merciful. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

The soul’s “progress and success, in all the worlds of God” is dependent on the virtue of ”truthfulness”

Truthfulness is the foundation of all human virtues. Without truthfulness progress and success, in all the worlds of God, are impossible for any soul. When this holy attribute is established in man, all the divine qualities will also be acquired. 
(‘Abdu’l-Baha, quoted by Shoghi Effendi, ‘The Advent of Divine Justice’)

Prayer is the “spiritual nourishment” of the soul

It is the soul of man that has first to be fed. And this spiritual nourishment prayer can best provide. 
(From a letter dated 8 December 1935 written on behalf of Shoghi Effendi to an individual believer, published in "Bahá'í News" 102 (August 1936), p. 3; The Compilation of Compilations, Vol. II, The Importance of Prayer, Meditation and the Devotional Attitude)

”The art of music is divine and effective. It is the food of the soul and spirit.”

What a wonderful meeting this is! These are the children of the Kingdom. The song we have just listened to was very beautiful in melody and words. The art of music is divine and effective. It is the food of the soul and spirit. Through the power and charm of music the spirit of man is uplifted. It has wonderful sway and effect in the hearts of children, for their hearts are pure, and melodies have great influence in them. The latent talents with which the hearts of these children are endowed will find expression through the medium of music. Therefore, you must exert yourselves to make them proficient; teach them to sing with excellence and effect. It is incumbent upon each child to know something of music, for without knowledge of this art the melodies of instrument and voice cannot be rightly enjoyed. Likewise, it is necessary that the schools teach it in order that the souls and hearts of the pupils may become vivified and exhilarated and their lives be brightened with enjoyment. 
(‘Abdu’l-Baha, from a talk at Children’s Reception Studio Hall in Washington, D.C., 24 April, 1912; ‘The Promulgation of Universal Peace: Talks Delivered by Abdu'l-Bahá during His Visit to the United States and Canada in 1912’)

Human voice “connects itself with the spirit” and “the spirit can be uplifted” by human voice

In the world of existence physical things have a connection with spiritual realities. One of these things is the voice, which connects itself with the spirit; and the spirit can be uplifted by this means -- for though it is a physical thing, it is one of the material, natural organizations -- therefore, it is effective. 
('Abdu'l-Bahá's, from an address quoted by to Mary L. Lucas in "A Brief Account of My Visit to Acca"; The Compilation of Compilations vol II) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)

“As the soul of man is not the result of composition and does not come into being through the affinity of molecular elements, it is not subject to disintegration.”

We must first prove that there is no annihilation in creation. Annihilation is only the decomposition of elements. For example, all these things we see in existence are made up of elements; that is, single atoms have combined and have formed infinite patterns. Every combination produces an entity. For example, through the combination of certain elements this flower has come into existence. Its annihilation means only the decomposition of this combination, it does not mean the destruction of the individual atoms or principal elements because these remain and will not be destroyed. When we say that this flower is destroyed we mean that its combination is decomposed but the principal elements remain; only their combination is destroyed.

In the same way, man has come into existence as the result of the combination of certain elements. Although his death is the disintegration of these elements, the elements are not annihilated. Therefore, life consists of the combination of elements and death of their dissolution or transference from one state to another. The transference of the vegetable to the animal world is the death of the former. Draw your own conclusions from this. Similarly, the transference of man from this world of matter and the dissolution of his elements constitute human death. Thus, it should be known that for existence there is no death. At most there is a transfer from one state to another. As the soul of man is not the result of composition and does not come into being through the affinity of molecular elements, it is not subject to disintegration. If it were, then we would say the soul has died. But because it is not composed, therefore it cannot be decomposed. And it is clear that even the basic elements are not subject to decomposition as they are not a compound composition. No doubt remains regarding this matter. (‘Abdu’l-Baha, from a talk in Dublin, USA, August 7, 1912; Mahmud’s Diary)

There is a “spiritual connection” between “the spirit of man and the world of existence”

It is obvious that a very deep relationship exists between the spirit of man and the world of existence. It often happens that what we see in a dream comes to pass in wakefulness; indeed, even in wakefulness when the soul is unfettered, matters come to mind which later appear in visible form. This shows that between the reasoning power of man and that visible appearance there is a spiritual connection. Furthermore, the spirit of man has the power of discovering facts. When this power is realized, problems become easily resolved. Amusingly, while materialists are engaged in making discoveries concerning the strata of the earth, they are immersed in the very spiritual power whose potency and influence they deny. They also claim to forecast coming events. 
(From a conversation by ‘Abdu’l-Baha, June 10, 1912, New York, ‘Mahmud’s Diary’)

One needs to “purify and sanctify” his/her “spirit from the impurities of the world of nature”

The principle of baptism is purification by repentance. John admonished and exhorted the people, and caused them to repent; then he baptized them. Therefore, it is apparent that this baptism is a symbol of repentance from all sin: its meaning is expressed in these words: “O God! as my body has become purified and cleansed from physical impurities, in the same way purify and sanctify my spirit from the impurities of the world of nature, which are not worthy of the Threshold of Thy Unity!” Repentance is the return from disobedience to obedience. Man, after remoteness and deprivation from God, repents and undergoes purification: and this is a symbol signifying “O God! make my heart good and pure, freed and sanctified from all save Thy love.”

As Christ desired that this institution of John should be used at that time by all, He Himself conformed to it in order to awaken the people and to complete the law of the former religion. Although the ablution of repentance was the institution of John, it was in reality formerly practiced in the religion of God.

Christ was not in need of baptism; but as at that time it was an acceptable and praiseworthy action, and a sign of the glad tidings of the Kingdom, therefore, He confirmed it. However, afterward He said the true baptism is not with material water, but it must be with spirit and with water. In this case water does not signify material water, for elsewhere it is explicitly said baptism is with spirit and with fire, from which it is clear that the reference is not to material fire and material water, for baptism with fire is impossible. 
(‘Abdu’l-Baha, ‘Some Answered Questions’)

Man needs the assistance of the Holy Spirit to “free himself from the rage of the carnal passions” and become “filled with the eternal bounty”

Man cannot free himself from the rage of the carnal passions except by the help of the Holy Spirit. That is why He[Christ] says baptism with the spirit, with water and with fire is necessary, and that it is essential—that is to say, the spirit of divine bounty, the water of knowledge and life, and the fire of the love of God. Man must be baptized with this spirit, this water and this fire so as to become filled with the eternal bounty. Otherwise, what is the use of baptizing with material water? No, this baptism with water was a symbol of repentance, and of seeking forgiveness of sins.

But in the cycle of Bahá’u’lláh there is no longer need of this symbol; for its reality, which is to be baptized with the spirit and love of God, is understood and established. 
(‘Abdu’l-Baha, ‘Some Answered Questions’)

The existence of “spirit” in mineral and vegetable

As to the existence of spirit in the mineral: it is indubitable that minerals are endowed with a spirit and life according to the requirements of that stage. This unknown secret, too, hath become known unto the materialists who now maintain that all beings are endowed with life, even as He saith in the Qur’án, ‘All things are living’.

In the vegetable world, too, there is the power of growth, and that power of growth is the spirit. In the animal world there is the sense of feeling, but in the human world there is an all-embracing power. In all the preceding stages the power of reason is absent, but the soul existeth and revealeth itself. The sense of feeling understandeth not the soul, whereas the reasoning power of the mind proveth the existence thereof. 
(‘Abdu’l-Baha, ‘Tablet to August Forel)

The dependence of human spirit on the divine Spirit

O maid-servant of God! The human spirit hath an impression and effect in the world, but the divine Spirit giveth life to the souls and conferreth eternal life upon those who are attracted to the fragrances of God. This is the Great Cause from the Kingdom of thy Lord.

Be attached to this New Spirit which hath effective power in the realities of things and creates new creatures. 
(‘Abdu’l-Baha, ‘Tablets of ‘Abdu’l-Baha’, vol. 1) 

One’s “spirit” needs to be “healed” as part of “true healing”

All true healing comes from God! There are two causes for sickness, one is material, the other spiritual. If the sickness is of the body, a material remedy is needed, if of the soul, a spiritual remedy.

If the heavenly benediction be upon us while we are being healed then only can we be made whole, for medicine is but the outward and visible means through which we obtain the heavenly healing. Unless the spirit be healed, the cure of the body is worth nothing. All is in the hands of God, and without Him there can be no health in us! 
(‘Abdu’l-Baha, from a talk, October 19, 1911, Paris; ‘Paris Talks’) 

“man must be born again” -- "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit" (Christ)

Christ announced, "That which is born of the flesh is flesh; and that which is born of the Spirit is spirit," meaning that man must be born again. As the babe is born into the light of this physical world, so must the physical and intellectual man be born into the light of the world of Divinity. In the matrix of the mother the unborn child was deprived and unconscious of the world of material existence, but after its birth it beheld the wonders and beauties of a new realm of life and being. In the world of the matrix it was utterly ignorant and unable to conceive of these new conditions, but after its transformation it discovers the radiant sun, trees, flowers and an infinite range of blessings and bounties awaiting it. In the human plane and kingdom man is a captive of nature and ignorant of the divine world until born of the breaths of the Holy Spirit out of physical conditions of limitation and deprivation. Then he beholds the reality of the spiritual realm and Kingdom, realizes the narrow restrictions of the mere human world of existence and becomes conscious of the unlimited and infinite glories of the world of God. Therefore, no matter how man may advance upon the physical and intellectual plane, he is ever in need of the boundless virtues of Divinity, the protection of the Holy Spirit and the face of God. 
(‘Abdu'l-Baha, ‘The Promulgation of Universal Peace’, Talks Delivered by ‘Abdu'l-Bahá during His Visit to the United States and Canada in 1912’)

The Prophets come to this world to help us educate our own souls

The Prophets have appeared in this world with the mission that human souls may become the expressions of the Merciful, that they may be educated and developed, attain to love and amity and establish peace and agreement. 
(‘Abdu’l-Baha, excerpt from a talk given on 21 April 1912 in Washington D.C.; The Promulgation of Universal Peace) (To read the entire talk please visits Talks of ‘Abdu’l-Baha)

“The human spirit is a center of all virtues.”

The human spirit is a center of all virtues. The human spirit is a reflector of the bounties of God. The human spirit is destined to advance and to be enveloped with the effulgence of God, because the human spirit is immortal. (‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)

The human spirit is “independent of the body”

An evidence of the immortality of the human spirit is the fact that it is not affected by changes in the body. The body is subject to various conditions. At one time it may be healthy; at another time it may be susceptible to some pathology. At one time it may be weak or ill; at another time it may be well and strong; or a member may become lost. However, the reality within the body -- the human spirit or mind -- is the same. If the body should become lean the spirit would not become lean. If the body should become corpulent the spirit would not become corpulent. If the body should be dismembered the spirit would not be affected. In brief, the spirit is independent of the body, and the disintegrations that take place in the body do not affect the soul, for the human reality or spirit is ever the same.

Another indication that the spirit is distinct from the body is the fact that though the body be slumbering the spirit of man is awake. When man is asleep the body is as dead; the eye does not see, the ear does not hear, the tongue does not speak; all the physical forces are quiescent, are as dead. Notwithstanding this, the spirit sees, hears, converses, discovers realities and latent mysteries. Though the physical functions of the body be impaired the spirit is not affected. Though the body become incapacitated the spirit remains intact. Though the body be paralyzed the spirit is not paralyzed. 

(‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)

A proof concerning the immortality of the soul of each Prophet

Among other evidences of the immortality of the spirit is the fact that there can be no effect without a cause. It is impossible for a non-existent cause to produce existent effects. For instance, if we suppose the sun to be non-existent, it would logically follow that its rays and heat would be non-existent. However, we know that the solar energy does exist, because its rays and heat are continuous. The sun being the cause, and the rays and heat the effects, how could the cause be non-existent when the effects are continuous? Likewise, the effects of the Sun of Reality are continuous in the world of reality, for, inasmuch as the influence of the manifestations of God who lived thousands of years ago continues to be felt in the world of to-day, surely the divine manifestations themselves must somewhere be still existent. For example, His Holiness Christ lived upon the earth nineteen hundred years ago, yet the effects of his presence at that time are evident in human minds and hearts to-day. Then how could he be non-existent when his lights are still shining, his praises still sung? He is existent and immortal. The Christ is everlasting, even as its traces are everlasting. 
(‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of‘Abdu’l-Baha)

The fact that we can consult with our own spirit is a proof of the immortality of the spirit

There is another indication that we might mention concerning the immortality of the spirit. When man is confronted with a difficulty he seems to consult himself, as though there were a something within him, a power, a voice. He says, shall I do this, or shall I not do it? What shall be the consequence? What harm will result? Then from that consultant emanates a certain opinion, and that still small voice says that the results will be either favorable or unfavorable, and advises that the matter should or should not be carried out. What is that something man consults when he thus deliberates? It is undoubtedly the spirit, which is changeless and therefore immortal. 
(‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of ‘Abdu’l-Baha) 

“The worst of souls is he who causes hearts to be agitated and who becomes the cause of sadness.”

I hope that you will be under the protection of God, will succeed in rendering service to humanity and will always be a source of happiness to every heart. The best person is he who wins all hearts and is not the cause of grief to anyone. The worst of souls is he who causes hearts to be agitated and who becomes the cause of sadness. Always endeavor to make people happy and their hearts joyful so that you may become the cause of guidance to mankind. Proclaim the Word of God and diffuse the divine fragrances. (‘Abdu’l-Baha, from a talk, Denver, Colorado, September 26, 1912; Mahmud’s Diary)

“purified and sanctified” souls enter the “Divine Realm” after death

The center of the Sun of Truth and of the Supreme World is the Divine Kingdom. Those souls who are purified and sanctified, after the disintegration of this elemental tabernacle, hasten to the Divine Realm, and that Realm is in this universe, but the people of this world have no knowledge of it, just as the mineral and vegetable kingdoms have no knowledge of the animal and human kingdoms. (‘Abdu’l-Baha, from a Tablet to Mr. and Mrs. Ostburg, Boston, Mass; Star of the West, vol. 2, no. 5, June 5, 1911)

How a human soul could become like “a divine physician to mankind”

If in this day a soul shall act according to the precepts and the counsels of God, he will serve as a divine physician to mankind, and like the trump of Isráfíl [1], he will call the dead of this contingent world to life; for the confirmations of the Abhá Realm are never interrupted, and such a virtuous soul hath, to befriend him, the unfailing help of the Company on high. Thus shall a sorry gnat become an eagle in the fulness of his strength, and a feeble sparrow change to a royal falcon in the heights of ancient glory.

Wherefore, look not on the degree of your capacity, ask not if you are worthy of the task: rest ye your hopes on the help and loving-kindness, the favours and bestowals of Bahá’u’lláh—may my soul be offered up for His friends! Urge on the steed of high endeavour over the field of sacrifice, and carry away from this wide arena the prize of divine grace. (‘Abdu’l-Baha, ‘Selections from the Writings of ‘Abdu’l-Baha’)

[1] Believed to be the angel appointed to sound the trumpet on the Day of Resurrection to raise the dead at the bidding of the Lord.

The “human soul comes into being at the time of conception…”

... the Bahá'í Writings affirm that the human soul comes into being at the time of conception. However, they do not clearly define the exact biological moment and nature of the event described as conception and this may, indeed, be a question that is insoluble by human thought or investigation, since it relates to mysteries of the spiritual world and the nature of the soul itself. (From a letter dated 6 February 1997 written on behalf of the Universal House of Justice to a National Spiritual Assembly; compilation ‘Reproduction and other Biological Subjects ‘, prepared by the Research Department of the Universal House of Justice)

The light that pure, sanctified, and detached souls radiate after their separation from the body “is responsible for the progress of the world and the advancement of its peoples”

The nature of the soul after death can never be described, nor is it meet and permissible to reveal its whole character to the eyes of men. The Prophets and Messengers of God have been sent down for the sole purpose of guiding mankind to the straight Path of Truth. The purpose underlying Their revelation hath been to educate all men, that they may, at the hour of death, ascend, in the utmost purity and sanctity and with absolute detachment, to the throne of the Most High. The light which these souls radiate is responsible for the progress of the world and the advancement of its peoples. They are like unto leaven which leaveneth the world of being, and constitute the animating force through which the arts and wonders of the world are made manifest. Through them the clouds rain their bounty upon men, and the earth bringeth forth its fruits. (Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

The perception of the rational soul

The first condition of perception in the world of nature is the perception of the rational soul. In this perception and in this power all men are sharers, whether they be neglectful or vigilant, believers or deniers. This human rational soul is God's creation; it encompasses and excels other creatures; as it is more noble and distinguished, it encompasses things. The power of the rational soul can discover the realities of things, comprehend the peculiarities of beings, and penetrate the mysteries of existence. All sciences, knowledge, arts, wonders, institutions, discoveries and enterprises come from the exercised intelligence of the rational soul. 
(‘Abdu’l-Baha, "Some Answered Questions"; The Compilation of Compilations, vol. III, The Importance of Arts in Promoting the Faith)

The “Kingdom is the real world, and this nether place is only its shadow stretching out.”

Know thou that the Kingdom is the real world, and this nether place is only its shadow stretching out. A shadow hath no life of its own; its existence is only a fantasy, and nothing more; it is but images reflected in water, and seeming as pictures to the eye. 
(‘Abdu'l-Baha, ‘Selections from the Writings of Abdu'l-Baha’)

The “form” of the soul that is suitable for its immortality

When the soul attaineth the Presence of God, it will assume the form that best befitteth its immortality and is worthy of its celestial habitation. Such an existence is a contingent and not an absolute existence, inasmuch as the former is preceded by a cause, whilst the latter is independent thereof. Absolute existence is strictly confined to God, exalted be His glory. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

When “the souls that are not good die, they go entirely away from this earth and so cannot influence anyone.”

Question: Are there "earthbound" souls who try to have, and do have, an influence over people, sometimes taking entire possession of their wills?

'Abdu'l-Baha answered, "There are no earthbound souls. When the souls that are not good die, they go entirely away from this earth and so cannot influence anyone. They are spiritually dead. Their thoughts can have influence only while they are alive on the earth. Caiaphas had great influence during his life, but as soon as he died, his influence ceased. It was of this kind that Christ said, 'Let the dead bury their dead.' But the good souls are given eternal life, and sometimes God permits their thoughts to reach the earth to help the people." 
(From a table talk by ‘Abdu’l-Baha, ‘Daily Lessons Received at Akka January 1908”, by Helen Goodall and Ella Goodall Cooper)

The “soul is a sign of God, a heavenly gem …whose mystery no mind, however acute, can ever hope to unravel.”

Thou hast asked Me concerning the nature of the soul. Know, verily, that the soul is a sign of God, a heavenly gem whose reality the most learned of men hath failed to grasp, and whose mystery no mind, however acute, can ever hope to unravel. It is the first among all created things to declare the excellence of its Creator, the first to recognize His glory, to cleave to His truth, and to bow down in adoration before Him. If it be faithful to God, it will reflect His light, and will, eventually, return unto Him. If it fails, however, in its allegiance to its Creator, it will become a victim to self and passion, and will, in the end, sink in their depths. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

The effect of gossip and backbiting on one’s soul

"... Backbiting quencheth the light of the heart, and extinguisheth the life of the soul." 
(Bahá'u'lláh, 'The Kitáb-i-Íqán')  

“The human soul is an essence spiritual in entity and material or physical in function.”

The human soul is an essence spiritual in entity and material or physical in function. It is defined as essence because it is independent in itself, while the body is accidental and dependent upon the soul. 
(‘Abdu’l-Baha, from a talk, quoted by Lua Getsinger in her talk in San Francisco on December 17, 1911)(To see the transcript of the entire talk please visit Baha’i Talks, Messages and Articles)

The “human soul” is that “all-unifying agency” that connects “the various organs and members, the parts and elements, that constitute the body of man” and “causeth them to function in perfect harmony and with absolute regularity, thus making the continuation of life possible.”

By nature is meant those inherent properties and necessary relations derived from the realities of things. And these realities of things, though in the utmost diversity, are yet intimately connected one with the other. For these diverse realities an all-unifying agency is needed that shall link them all one to the other. For instance, the various organs and members, the parts and elements, that constitute the body of man, though at variance, are yet all connected one with the other by that all-unifying agency known as the human soul, that causeth them to function in perfect harmony and with absolute regularity, thus making the continuation of life possible. The human body, however, is utterly unconscious of that all-unifying agency, and yet acteth with regularity and dischargeth its functions according to its will. 
(‘Abdu’l-Baha, ‘Tablet to August Forel)

To train our soul in this life so that “at the hour of death, [it will] ascend, in the utmost purity and sanctity and with absolute detachment.”

The Prophets and Messengers of God have been sent down for the sole purpose of guiding mankind to the straight Path of Truth. The purpose underlying Their revelation hath been to educate all men, that they may, at the hour of death, ascend, in the utmost purity and sanctity and with absolute detachment, to the throne of the Most High. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

Vastness of next world compared to this world

The world beyond is as different from this world as this world is different from that of the child while still in the womb of its mother. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

Receiving “eternal life” from the divine Spirit

O maid-servant of God! The human spirit hath an impression and effect in the world, but the divine Spirit giveth life to the souls and conferreth eternal life upon those who are attracted to the fragrances of God. This is the Great Cause from the Kingdom of thy Lord.

Be attached to this New Spirit which hath effective power in the realities of things and creates new creatures. 
(‘Abdu’l-Baha, ‘Tablets of ‘Abdu’l-Baha’, vol. 1)

“…the soul, after its separation from the body, will continue to progress until it attaineth the presence of God…”

And now concerning thy question regarding the soul of man and its survival after death. Know thou of a truth that the soul, after its separation from the body, will continue to progress until it attaineth the presence of God, in a state and condition which neither the revolution of ages and centuries, nor the changes and chances of this world, can alter. It will endure as long as the Kingdom of God, His sovereignty, His dominion and power will endure. It will manifest the signs of God and His attributes, and will reveal His loving kindness and bounty. The movement of My Pen is stilled when it attempteth to befittingly describe the loftiness and glory of so exalted a station. The honor with which the Hand of Mercy will invest the soul is such as no tongue can adequately reveal, nor any other earthly agency describe. Blessed is the soul which, at the hour of its separation from the body, is sanctified from the vain imaginings of the peoples of the world. Such a soul liveth and moveth in accordance with the Will of its Creator, and entereth the all-highest Paradise. 
(Baha'u'llah, Gleanings from the Writings of Baha'u'llah)

“…physical ailments, no matter how severe, cannot bring any change in the inherent condition of the soul”

Regarding your questions concerning the condition of the soul during illness. The passages in the "Gleanings" make it quite clear that physical ailments, no matter how severe, cannot bring any change in the inherent condition of the soul. As Bahá'u'lláh says: 'The spirit is permanent and steadfast in its station'. The veil or hindrance that interposes between soul and body during physical disease is sickness itself. Sickness reveals itself. Sickness reveals a lack of balance in human organism, an absence of equilibrium in the forces essential for the normal functioning of the human body. 
(From a letter written on behalf of Shoghi Effendi to an individual believer, March 8, 1936; Lights of Guidance)

We need to free our souls from the “bondage of self”

Deliver your souls, O people, from the bondage of self, and purify them from all attachment to anything besides Me. Remembrance of Me cleanseth all things from defilement, could ye but perceive it. 
(Baha’u’llah, ‘Gleanings from the Writings of Bahá'u'lláh’; The Compilation of Compilations, vol. II, The Importance of Prayer, Meditation, and the Devotional Attitude)

“…the human soul is exalted above all egress and regress”

In several of Our Tablets We have referred to this theme, and have set forth the various stages in the development of the soul. Verily I say, the human soul is exalted above all egress and regress. It is still, and yet it soareth; it moveth, and yet it is still. It is, in itself, a testimony that beareth witness to the existence of a world that is contingent, as well as to the reality of a world that hath neither beginning nor end. Behold how the dream thou hast dreamed is, after the lapse of many years, re-enacted before thine eyes. Consider how strange is the mystery of the world that appeareth to thee in thy dream. Ponder in thine heart upon the unsearchable wisdom of God, and meditate on its manifold revelations.... 
(Baha’u’llah, 'Gleanings from the Writings of Baha’u’llah'; Compilation ‘Reincarnation and the Nature and Progress of the Soul’, prepared by the Research Department of the Universal House of Justice)

The soul is God’s Sanctuary -- reason is God’s Throne

The soul is the Sanctuary of God; Reason is His Throne. 
(Words of ‘Abdu’l-Baha, recorded by Julia Grundy during her pilgrimage, ‘Ten Days in the Light of Acca’)

“The personality and activity of the body are due to the individuality of the soul.”

The personality and activity of the body are due to the individuality of the soul. Thus the soul is the cause of the life of the body, and the body, with all its organs, is but the vehicle of its expression. 
(‘Abdu’l-Baha, from a talk,  quoted by Lua Getsinger in her talk in San Francisco on December 17, 1911)(To see the transcript of the entire talk please visit Baha’i Talks, Messages and Articles)

The world of dream is a proof of the existence of the soul

… the human soul is exalted above all egress and regress. It is still, and yet it soareth; it moveth, and yet it is still. It is, in itself, a testimony that beareth witness to the existence of a world that is contingent, as well as to the reality of a world that hath neither beginning nor end. Behold how the dream thou hast dreamed is, after the lapse of many years, re-enacted before thine eyes. Consider how strange is the mystery of the world that appeareth to thee in thy dream. Ponder in thine heart upon the unsearchable wisdom of God, and meditate on its manifold revelations.... 
(Baha’u’llah, Gleanings from the Writings of Baha’u’llah; Compilation ‘Reincarnation and the Nature and Progress of the Soul’, prepared by the Research Department of the Universal House of Justice)

“..Mental faculties .. are in truth of the inherent properties of the soul ..”

Now concerning mental faculties, they are in truth of the inherent properties of the soul, even as the radiation of light is the essential property of the sun. (‘Abdu’l-Baha, ‘Tablet to August Forel’)

Government of one's self and soul

Moral life consists in the government of one's self. Immortality is government of a human soul by the Divine Will. 
(Words of ‘Abdu’l-Baha, recorded by Julia Grundy during her pilgrimage, ‘Ten Days in the Light of Acca’)

The “soul of man is a ray of light emanating from the Sun of the Word of God”

Therefore, since the reality or soul of man is a ray of light emanating from the Sun of the Word of God, it is capable of manifesting all the perfections of Being. It is then worthy to be the throne upon which may be established the manifestation of the Names, the Qualities and the Attributes of the Ancient Entity. Inasmuch as this entity is capable of expressing the great virtues of Being, and is the greatest and purest of all existing beings on earth, from its earliest rise in the human temple to its resurrection from the grave of the body, it can be likened to a mirror which reveals the seen and the unseen, which possesses the virtues of the world and of the Kingdom. 
('Abdu'l-Baha, from a talk, quoted by Lua Getsinger in her talk in San Francisco on December 17, 1911)(To see the transcript of the entire talk please visit Baha’i Talks, Messages and Articles)