Question. - What is the meaning of the Trinity, of the Three Persons in One?
Answer. - The Divine Reality, which is purified and sanctified from the understanding of human beings and which can never be imagined by the people of wisdom and of intelligence, is exempt from all conception. That Lordly Reality admits of no division; for division and multiplicity are properties of creatures which are contingent existences, and not accidents which happen to the self-existent.
The Divine Reality is sanctified from singleness, then how much more from plurality. The descent of that Lordly Reality into conditions and degrees would be equivalent to imperfection and contrary to perfection, and is, therefore, absolutely impossible. It perpetually has been, and is, in the exaltation of holiness and sanctity. All that is mentioned of the Manifestations and Dawning-places of God signifies the divine reflection, and not a descent into the conditions of existence.
God is pure perfection, and creatures are but imperfections. For God to descend into the conditions of existence would be the greatest of imperfections; on the contrary, His manifestation, His appearance, His rising are like the reflection of the sun in a clear, pure, polished mirror. All the creatures are evident signs of God, like the earthly beings upon all of which the rays of the sun shine. But upon the plains, the mountains, the trees and fruits, only a portion of the light shines, through which they become visible, and are reared, and attain to the object of their existence, while the Perfect Man is in the condition of a clear mirror in which the Sun of Reality becomes visible and manifest with all its qualities and perfections...
...So the Reality of Christ was a clear and polished mirror of the greatest purity and fineness. The Sun of Reality, the Essence of Divinity, reflected itself in this mirror and manifested its light and heat in it; but from the exaltation of its holiness, and the heaven of its sanctity, the Sun did not descend to dwell and abide in the mirror. No, it continues to subsist in its exaltation and sublimity, while appearing and becoming manifest in the mirror in beauty and perfection. Now if we say that we have seen the Sun in two mirrors - one the Christ and one the Holy Spirit - that is to say, that we have seen three Suns, one in heaven and the two others on the earth, we speak truly. And if we say that there is one Sun, and it is pure singleness, and has no partner and equal, we again speak truly...
...The epitome of the discourse is that the Reality of Christ was a clear mirror, and the Sun of Reality - that is to say, the Essence of Oneness, with its infinite perfections and attributes - became visible in the mirror. The meaning is not that the Sun, which is the Essence of the Divinity, became divided and multiplied - for the Sun is one - but it appeared in the mirror. This is why Christ said, "The Father is in the Son," meaning that the Sun is visible and manifest in this mirror.
The Holy Spirit is the Bounty of God which becomes visible and evident in the Reality of Christ. The Sonship station is the heart of Christ, and the Holy Spirit is the station of the spirit of Christ. Hence it has become certain and proved that the Essence of Divinity is absolutely unique and has no equal, no likeness, no equivalent. This is the signification of the Three Persons of the Trinity. If it were otherwise, the foundations of the Religion of God would rest upon an illogical proposition which the mind could never conceive, and how can the mind be forced to believe a thing which it cannot conceive? A thing cannot be grasped by the intelligence except when it is clothed in an intelligible form; otherwise, it is but an effort of the imagination.
It has now become clear, from this explanation, what is the meaning of the Three Persons of the Trinity. The Oneness of God is also proved.
-`Abdu'l-Baha: Some Answered Questions, pages 113-115