The parable of the lost sheep speaks graphically and vividly of the purpose of the coming of the Son of God into the world. The good shepherd leaves the ninety-nine sheep, by which is meant the angelic world, and sets out for the mountains in order to seek out his lost sheep - the human race perishing in sins. The shepherd's great love for the perishing sheep is evident not only in the fact that he solicitously seeks it, but especially in the fact that after finding it, he takes it upon his shoulders and carries it back. In other words, God, by His power, returns to man the innocence, holiness and blessedness lost by him; having united Himself with our human nature, the Son of God, according to the word of the Prophet, hath borne our griefs, and carried our sorrows (Isaiah, Ch. 53).
Christ became man not only to teach us the true way and to show us a good example. He became man in order to unite us with Himself, to join our feeble, diseased human nature to His divinity. The Nativity of Christ testifies to the fact that we attain the ultimate aim of our life not only by faith and by striving for good, but chiefly by the regenerating power of the incarnate Son of God, with Whom we are united.
Delving deeply into the mystery of the incarnation of the Son of God, we see that it is closely bound up with the mystery of Holy Communion and with the Church, which, according to apostolic teaching, is the mystical Body of Christ. In the Holy Communion of the Body and Blood of Christ, a man is joined to the divine-human nature of Christ; he unites with Him and in this union is wholly transfigured. At the same time, in Holy Communion, a Christian unites also with other members of the Church - and thus the mystical Body of Christ grows.
Heterodox Christians who do not believe in Holy Communion understand union with Christ in an allegorical, metaphorical sense, or in the sense of only a spiritual communion with Him. But for spiritual communion, the incarnation of the Son of God is superfluous. After all, even before the Nativity of Christ, the prophets and the righteous were counted worthy of grace-filled communion with God.
One must understand that man is ill not only spiritually, but also physically: all of human nature has been harmed by sin. It is essential, therefore, to heal the whole man, not only his spiritual part. To remove any doubt in the necessity for total communion with Himself, the Lord Jesus Christ, in His discourse on the Bread of Life, speaks thus: Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you. Whoso eateth my flesh, and drinketh my blood, hath eternal life; and I will raise him up at the last day... He that eateth my flesh, and drinketh my blood, dwelleth in me, and I in him (John 6:53-54, 56). Later, Christ uses the metaphor of the grapevine to explain to His disciples that it is precisely in close union with Him that man receives the strength essential for spiritual development and perfection: As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me. I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing (John 15:4-5).
Some holy Fathers have justly likened Holy Communion to the mystical tree of life, from which our primogenitors ate in Eden, and which afterwards St. John the Theologian saw in paradise (Gen. 2:9, Rev. 2:7, 22:2). In Holy Communion, a Christian is joined to the immortal life of the God-Man.
Thus, the purpose of the incarnation of the Son of God lies in the spiritual and physical regeneration of man. Spiritual renewal is accomplished throughout the course of a Christian's whole life. But the renewal of his physical nature is completed on the day of the general resurrection of the dead, when the righteous shall shine forth as the sun in the kingdom of their Father (Matt. 13:43).