There is an intermediary idea, a more mystical view of Avatarhood which supposes that a human soul calls down this descent into himself and is either possessed by the divine consciousness or becomes an effective reflection or channel of it. This view rests upon certain truths of spiritual experience. The divine birth in man, his ascent, is itself a growing of the human into the divine consciousness, and in its intensest culmination is a losing of the separate self in that. The soul merges its individuality in an infinite and universal being or loses it in the heights of a transcendent being; it becomes one with the Self, the Brahman, the Divine or, as it is sometimes more absolutely put, becomes the one Self, the Brahman, the Divine. The Gita itself speaks of the soul becoming the Brahman, brahmabhuta, and of its there by dwelling in the Lord, in Krishna, but it does not, it must be marked, speak of it as becoming the Lord or the Purushottama, though it does declare that the Jiva himself is always Ishwara, the partial being of the Lord, mamaivanshah. For this greatest union, this highest becoming is still part of the ascent; while it is the divine birth to which every Jiva arrives, it is not the descent of the Godhead, not Avatarhood, but at most Buddhahood according to the doctrine of the Buddhists, it is the soul awakened from its present mundane individuality into an infinite super-consciousness. That need not carry with it either the inner consciousness or the characteristic action of the Avatar.
On the other hand, this entering into the divine consciousness may be attended by a reflex action of the Divine entering or coming forward into the human parts of our being, pouring himself into the nature, the activity, the mentality, the corporeality even of the man; and that may well be at least a partial Avatarhood. The Lord stands in the heart, says the Gita,—by which it means of course the heart of the subtle being, the nodus of the emotions, sensations, mental consciousness, where the individual Purusha also is seated,—but he stands there veiled, enveloped by his Maya. But above, on a plane within us but now superconscient to us, called heaven by the ancient mystics, the Lord and the Jiva stand together revealed as of one essence of being, the Father and the Son of certain symbolisms, the Divine Being and the divine Man who comes forth from Him born of the higher divine Nature, the virgin Mother, para prakriti, para maya, into the lower or human nature. This seems to be the inner doctrine of the Christian incarnation; in its Trinity the Father is above in this inner Heaven; the Son or supreme Prakriti become Jiva of the Gita descends as the divine Man upon earth, in the mortal body; the Holy Spirit, pure Self, Brahmic consciousnessis that which makes them one and that also in which they communicate; for we hear of the Holy Spirit descending upon Jesus and it is the same descent which brings down the powers of the higher consciousness into the simple humanity of the Apostles.
N.B:- In the Buddhist legend the name of the mother of Buddha makes the symbolism clear; in the Christian the symbol seems to have been attached by a familiar mythopoeic process to the actual human mother of Jesus of Nazareth.
But also the higher divine consciousness of the Purushottama may itself descend into the humanity and that of the Jiva disappear into it. This is said by his contemporaries to have happened in the occasional transfigurations of Chaitanya when he who in his normal consciousness was only the lover and devotee of the Lord and rejected all deification, became in these abnormal moments the Lord himself and so spoke and acted, with all the outflooding light and love and power of the divine Presence. Supposing this to be the normal condition, the human receptacle to be constantly no more than a vessel of this divine Presence and divine Consciousness, we should have the Avatar according to this intermediary idea of the incarnation. That easily recommends itself as possible to our human notions; for if the human being can elevate his nature so as to feel a unity with the being of the Divine and himself a mere channel of its consciousness, light, power, love, his own will and personality lost in that will and that being,—and this is a recognised spiritual status,—then there is no inherent impossibility of the reflex action of that Will, Being, Power, Love, Light, Consciousness occupying the whole personality of the human Jiva. And this would not bemerely an ascent of our humanity into the divine birth and the divine nature, but a descent of the divine Purusha into humanity, an Avatar.
The Gita, however, goes much farther. It speaks clearly of the Lord himself being born; Krishna speaks of his many births that are past and makes it clear by his language that it is not merely the receptive human being but the Divine of whom he makes this affirmation, because he uses the very language of the Creator, the same language which he will employ when he has to describe his creation of the world. “Although I am the unborn Lord of creatures, I create (loose forth) my self by my Maya,” presiding over the actions of my Prakriti. Here there is no question of the Lord and the human Jiva or of the Father and the Son, the divine Man, but only of the Lord and his Prakriti. The Divine descends by his own Prakriti into birth in its human form and type and brings into it the divine Consciousness and the divine Power, though consenting, though willing to act in the form, type, mould of humanity, and he governs its actions in the body as the indwelling and over-dwelling Soul, adhisthaya. From above he governs always, indeed, for so he governs all nature, the human included; from within also he governs all nature, always, but hidden; the difference here is that he is manifest, that the nature is conscious of the divine Presence as the Lord, the Inhabitant, and it is not by his secret will from above, “the will of the Father which is in heaven,” but by his quite direct and apparent will that he moves the nature. And here there seems to be no room for the human intermediary; for it is by resort to his own nature, prakritim svam, and not the special nature of the Jiva that the Lord of all existence thus takes upon himself the human birth.
Reference:- Essays on Gita by Sri Aurobindo