Why the Nativity? Why the Incarnation? Here is what we have believed from the beginning regarding these questions.
“The Word of God, our Lord Jesus Christ, who did, through His transcendent love, become what we are, that He might bring us to be even what He is Himself.” Irenaeus (c. 130-200), Against Heresies, Book 5, Preface
“The Word of God became man, that thou mayest learn from man how man may become God.” St. Clement of Alexandria (150-215), Exhortation to the Heathen, Chapter I
“Therefore He was not man, and then became God, but He was God, and then became man, and that to deify us.” St. Athanasius (296-373), Against the Arians, Discourse I, Paragraph 39
“For He was made man that we might be made God.” St. Athanasius (296-373), On the Incarnation, Section 54
“For just as He in Himself assimilated His own human nature to the power of the Godhead, being a part of the common nature, but not being subject to the inclination to sin which is in that nature (for it says: “He did no sin, nor was deceit found in his mouth), so, also, will He lead each person to union with the Godhead if they do nothing unworthy of union with the Divine.” St. Gregory of Nyssa (c. 335-395), On Christian Perfection
“That which He has not assumed He has not healed; but that which is united to His Godhead is also saved. If only half Adam fell, then that which Christ assumes and saves may be half also; but if the whole of his nature fell, it must be united to the whole nature of Him that was begotten, and so be saved as a whole.” St. Gregory of Nazianzus (321-390), Epistle 101
“This Logos the Father in the latter days sent forth, no longer to speak by a prophet, and not wishing that the Word, being obscurely proclaimed, should be made the subject of mere conjecture, but that He should be manifested, so that we could see Him with our own eyes. This Logos, I say, the Father sent forth, in order that the world, on beholding Him, might reverence Him who was delivering precepts not by the person of prophets, nor terrifying the soul by an angel, but who was Himself—He that had spoken—corporally present amongst us. This Logos we know to have received a body from a virgin, and to have remodelled the old man by a new creation. And we believe the Logos to have passed through every period in this life, in order that He Himself might serve as a law for every age, and that, by being present (amongst) us, He might exhibit His own manhood as an aim for all men. And that by Himself in person He might prove that God made nothing evil, and that man possesses the capacity of self-determination, inasmuch as he is able to will and not to will, and is endued with power to do both. This Man we know to have been made out of the compound of our humanity. For if He were not of the same nature with ourselves, in vain does He ordain that we should imitate the Teacher. For if that Man happened to be of a different substance from us, why does He lay injunctions similar to those He has received on myself, who am born weak; and how is this the act of one that is good and just? In order, however, that He might not be supposed to be different from us, He even underwent toil, and was willing to endure hunger, and did not refuse to feel thirst, and sunk into the quietude of slumber. He did not protest against His Passion, but became obedient unto death, and manifested His resurrection. Now in all these acts He offered up, as the first-fruits, His own manhood, in order that thou, when thou art in tribulation, mayest not be disheartened, but, confessing thyself to be a man (of like nature with the Redeemer), mayest dwell in expectation of also receiving what the Father has granted unto this Son.” Saint Hippolytus of Rome (?-c.235), The Refutation of all the heresies, Book 10, Chapter 29