According to Sri Aurobindo Christ very much fits into the scheme of Avatars who came to earth with a special mission from the Divine. He based this not on any belief system nor through any religious faith in Christianity but on the intuitive knowledge of truths that are not normally accessible to men. Here is what he writes about the series of Avatars:
For there are two aspects of the divine birth; one is a descent, the birth of God in humanity, the Godhead manifesting itself in the human form and nature, the eternal Avatar; the other is an ascent, the birth of man into the Godhead, man rising into the divine nature and consciousness, madbhavam agatah; it is the being born anew in a second birth of the soul. It is that new birth which Avatarhood and the upholding of the Dharma are intended to serve. This double aspect in the Gita’s doctrine of Avatarhood is apt to be missed by the cursory reader satisfied, as most are, with catching a superficial view of its profound teachings, and it is missed too by the formal commentator petrified in the rigidity of the schools. Yet it is necessary, surely, to the whole meaning of the doctrine. Otherwise the Avatar idea would be only a dogma, a popular superstition, or an imaginative or mystic deification of historical or legendary supermen, not what the Gita makes all its teaching, a deep philosophical and religious truth and an essential part of or step to the supreme mystery of all, rahasyam uttamam.
If there were not this rising of man into the Godhead to be helped by the descent of God into humanity, Avatarhood for the sake of the Dharma would be an otiose phenomenon, since mere Right, mere justice or standards of virtue can always be upheld by the divine omnipotence through its ordinary means, by great men or great movements, by the life and work of sages and kings and religious teachers, without any actual incarnation. The Avatar comes as the manifestation of the divine nature in the human nature, the apocalypse of its Christhood, Krishnahood, Buddhahood, in order that the human nature may by moulding its principle, thought, feeling, action, being on the lines of that Christhood, Krishnahood, Buddhahood transfigure itself into the divine. The law, the Dharma which the Avatar establishes is given for that purpose chiefly; the Christ, Krishna, Buddha stands in its centre as the gate, he makes through himself the way men shall follow. That is why each Incarnation holds before men his own example and declares of himself that he is the way and the gate; he declares too the oneness of his humanity with the divine being, declares that the Son of Man and the Father above from whom he has descended are one, that Krishna in the human body, manusım tanum asritam, and the supreme Lord and Friend of all creatures are but two revelations of the same divine Purushottama, revealed there in his own being, revealed here in the type of humanity. 
This is especially significant since the phenomenon of the Avatar has always been shrouded in mystery and though the term is now in much wider use and rather misused to signify a whole lot of phenomena, no one has written so extensively and with so much clarity on the subject as Sri Aurobindo. One may well understand this since not only did he have access to the highest supramental knowledge, as the Mother revealed, Sri Aurobindo is the last Avatar in a human body. He sums up in his single being what was foreseen as the coming of Kalki, the return of Christ, the reincarnation of Buddha as the Maitreyi Amitabh and of course the sambhavami yuge yuge of Sri Krishna. But this we can leave for later and return to Christ himself. However, Sri Aurobindo cautions us that instead of focusing on the historicity of great luminous divine beings what is important is to understand the inner spiritual truth that they represent so that drawing the needed strength and inspiration we may mould our life towards a growing image of divinity in man.
Such controversies as the one that has raged in Europe over the historicity of Christ, would seem to a spiritually-minded Indian largely a waste of time; he would concede to it a considerable historical, but hardly any religious importance; for what does it matter in the end whether a Jesus son of the carpenter Joseph was actually born in Nazareth or Bethlehem, lived and taught and was done to death on a real or trumped-up charge of sedition, so long as we can know by spiritual experience the inner Christ, live uplifted in the light of his teaching and escape from the yoke of the natural Law by that atonement of man with God of which the crucifixion is the symbol? If the Christ, God made man, lives within our spiritual being, it would seem to matter little whether or not a son of Mary physically lived and suffered and died in Judea. So too the Krishna who matters to us is the eternal incarnation of the Divine and not the historical teacher and leader of men. 
As we know the birth of Christ was amidst a humanity that was at least semi-barbaric in its ways. It was given to Christ to teach it the gentler ways of love and forgiveness. How else could he do it if he did not show the way by his own example of forgiving and loving even those who hated him? In this respect, he is so much like Buddha who was moved by the suffering of humanity and its selfish ignorant ways and brought to it a new and diviner way of life. That is what Sri Aurobindo says in his aphorism that Christ from his cross humanized Europe. The Mother reveals the truth about it thus:
When Christ came upon earth, he brought a message of brotherhood, love and peace. But he had to die in pain, on the cross, so that his message might be heard. For men cherish suffering and hatred and want their God to suffer with them. They wanted this when Christ came and, in spite of his teaching and sacrifice, they still want it; and they are so attached to their pain that, symbolically, Christ is still bound to his cross, suffering perpetually for the salvation of men. 
Subsequently, after the passing away of Christ, as is the story after the departure of every divine being who walks in a greater freedom and light than man has access to, there were two approaches to his teachings, if teachings we can say. One line picked up some words and adding to it its own darkness and smallness turned it into a dogmatic religion which is perhaps as opposed to the spirit of Christ as of the followers of any other religion. Sri Aurobindo reveals this to us:
The Christian martyrs perish in their thousands, setting soul-force against empire-force that Christ may conquer, Christianity prevail. Soul force does triumph, Christianity does prevail,—but not Christ; the victorious religion becomes a militant and dominant Church and a more fanatically persecuting power than the creed and the empire which it replaced. The very religions organise themselves into powers of mutual strife and battle together fiercely to live, to grow, to possess the world. 
On the other hand, another line tried to take up his teachings in the spirit of yoga, to use them as a means of their ascension. There are always in the teachings of the Masters mystic and esoteric elements and Christ is no exception. These followers had beautiful psychic and spiritual experiences and their writings are a testimony to that. For example, the assumption of the feast of Mary, the redemption of sin by his sacrifice, the cross itself are powerful mystic symbols. As revealed by Sri Aurobindo and The Mother thus:
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 consciousness is 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. 
The virgin Mary is clearly the Para Prakriti that is undefiled by the touch of lower nature. She becomes the Jiva, the soul in man.
The Cross is in Yoga the symbol of the soul and nature in their strong and perfect union, but because of our fall into the impurities of ignorance it has become the symbol of suffering and purification. 
As to the action of Christ it had, like all avatars, both a local and a global aspect. The Mother reveals:
In the Essays on the Gita Sri Aurobindo mentions the names of three Avatars, and Christ is one of them. An Avatar is an emanation of the Supreme Lord who assumes a human body on earth. I heard Sri Aurobindo himself say that Christ was an emanation of the Lord’s aspect of love.
The death of Caesar marked a decisive change in the history of Rome and the countries dependent on her. It was therefore an important event in the history of Europe. But the death of Christ was the starting-point of a new stage in the evolution of human civilisation. This is why Sri Aurobindo tells us that the death of Christ was of greater historical significance, that is to say, it has had greater historical consequences than the death of Caesar. The story of Christ, as it has been told, is the concrete and dramatic enactment of the divine sacrifice: the Supreme Lord, who is All-Light, All-Knowledge, All- Power, All-Beauty, All-Love, All-Bliss, accepting to assume human ignorance and suffering in matter, in order to help men to emerge from the falsehood in which they live and because of which they die. 
It is however a sad truth and perhaps also a reflection of the Age and the soil in which Christ’s teachings were sown, that the rigid religionists won over the spiritual and mystic elements. What followed is a horror tale of inquisition conquests and everything else all in the name of Christ who still hangs upon the Cross for a humanity that refuses to change its Asuric ways. But this is true of all religions and the lesser said the better it is about what humanity does to the luminous words of all Masters including that of Christ. In India however there always existed a natural corrective in the form of not only an acceptance of many approaches to the Divine but also different stages of spiritual evolution leading to the difference in the degree of revelation that a Master or even an Avatar brings for man. Another idea that saved India while keeping the flame of Sanatan Dharma alive is the lineage of Masters and Seers, Saints and Sages and the refusal to accept one book as final and ultimate. The fundamental truths of the Divine Reality do not change but there are many new aspects that come up in the course of Its evolutionary manifestation. Due to these unique aspects India always remained and continues to remain the custodian and guardian of the Sanatan Dharma which though universal in its scope and significance has always been kept alive in India both in its spirit and significance. Unfortunately persecuted by the orthodox conservative spirit mixed with politics took the life force of early Christianity leaving behind a dead and empty shell of the Church and the Institutions built in his name.
As to the date 25th December itself it is well known that it has nothing to do with Christ himself. It is a day that was long back celebrated as the Return of Light in Jewish as well as other traditions. The ancient traditions often linked religious life with significant events of earthly life. For example, both in January and April we see the celebration of New Year in certain parts of India linked to the sowing and reaping of the harvest. Similarly, we see the celebration of Saraswati Puja in spring since she is indeed the goddess of manifold aspects of creation. On the other hand, Kali Puja comes right towards the end of the year, the Deepawali Amavasya, the dark night representing the goddess whose stride changes Time. In Christianity too we see something similar though on a much lesser scale. Two symbols that it clearly adopted are the 25th of December and the Good shepherd. We know that the winter solstice falls around the 21st or 22nd December. From the next day, the sun begins to enter the ascendance leading to a progressive lengthening of the days. Quite naturally it becomes a powerful symbol of a quickening impulse towards a new life. Perhaps the Christian calendar may also have something to do with this ascendance. Similarly, the 6th of January is another powerful symbol when three wise men seem to have visited Christ and given gifts. It is the day of epiphany. The Mother revealed the significance of these gifts as follows:
(Significances of the gifts offered by the three Magi to Jesus at the time of his birth)
Gold: wealth of the world and supramental knowledge.
Frankincense: purification of the vital.
Myrrh: immortalisation of the body. 
Thus seen we can understand the story of Christ as one of the efforts to divinize matter by the Divine Descent in a human body. However, the earth was far from ready at that time. The birth and death of Christ however left quivering in matter some touch of the Divine Love sacrificing itself for earth and humanity. The original sin is the birth of soul in matter assuming a vital and mental sheath which are imperfect and coats of unconsciousness and falsehood. Each Avatar comes to clear off something of this coating through the Divine Sacrifice. It is in this context we have to see Sri Aurobindo as the last among this line of Avatars whose poornahuti, the complete sacrifice of the Divine in matter on the 5th December 1950 finally opened the doors of the Supramental World. The resurrection which is a symbol of the redemption of matter was complete. It is only now a matter of time that the final fulfilment of terrestrial life should take place for which each Avatar comes. As the Mother observes and reveals,
In the eternity of becoming, each Avatar is only the announcer, the forerunner of a more perfect realisation.
And yet men have always the tendency to deify the Avatar of the past in opposition to the Avatar of the future.
Now again Sri Aurobindo has come announcing to the world the realisation of tomorrow; and again his message meets with the same opposition as of all those who preceded him. But tomorrow will prove the truth of what he revealed and his work will be done. (((CWM 13: 22)))
And Sri Aurobindo reveals in Savitri referring to the story of Christ and sacrifice of all divine beings including himself to redeem the earth and shape the animal humanity into the divine humanity it is meant to become.
He who would save himself lives bare and calm;
He who would save the race must share its pain:
This he shall know who obeys that grandiose urge.
The Great who came to save this suffering world
And rescue out of Time’s shadow and the Law,
Must pass beneath the yoke of grief and pain;
They are caught by the Wheel that they had hoped to break,
On their shoulders they must bear man’s load of fate.
Heaven’s riches they bring, their sufferings count the price
Or they pay the gift of knowledge with their lives.
The Son of God born as the Son of man
Has drunk the bitter cup, owned Godhead’s debt,
The debt the Eternal owes to the fallen kind
His will has bound to death and struggling life
That yearns in vain for rest and endless peace.
Now is the debt paid, wiped off the original score.
The Eternal suffers in a human form,
He has signed salvation’s testament with his blood:
He has opened the doors of his undying peace.
The Deity compensates the creature’s claim,
The Creator bears the law of pain and death;
A retribution smites the incarnate God.
His love has paved the mortal’s road to Heaven:
He has given his life and light to balance here
The dark account of mortal ignorance.
It is finished, the dread mysterious sacrifice,
Offered by God’s martyred body for the world;
Gethsemane and Calvary are his lot,
He carries the cross on which man’s soul is nailed;
His escort is the curses of the crowd;
Insult and jeer are his right’s acknowledgment;
Two thieves slain with him mock his mighty death.
He has trod with bleeding brow the Saviour’s way.
He who has found his identity with God
Pays with the body’s death his soul’s vast light.
His knowledge immortal triumphs by his death. 
- Essays on the Gita CWSA 19: 148-149[↩]
- Essays on the Gita CWSA 19: 15[↩]
- On Thoughts and Aphorisms CWM 10: 4[↩]
- Essays on the Gita CWSA 19: 44[↩]
- Essays on the Gita CWSA 19: 163[↩]
- Essays Divine and Human CWSA 12: 168[↩]
- On Aphorisms CWM 10: 61[↩]
- [Words of the Mother III, CWM 15: 198][↩]
- [Savitri CWSA 33-34: 444-445][↩]