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)