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 (‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)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(‘Abdu’l-Baha, excerpt from a talk at Theosophical Society, San Francisco, October 11, 1912; Unpublished Manuscript by Ella Cooper) (To read the entire talk please visit Talks of ‘Abdu’l-Baha)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