St. Athanasius on the Incarnation

It was unthinkable that God, the Father of Truth, should go back upon His word regarding death in order to ensure our continued existence. He could not falsify Himself; what, then, was God to do? Was He to demand repentance from men for their transgression? You might say that that was worthy of God, and argue further that, as through the Transgression they became subject to corruption, so through repentance they might return to incorruption again. But repentance would not guard the Divine consistency, for, if death did not hold dominion over men, God would still remain untrue. Nor does repentance recall men from what is according to their nature; all that it does is to make them cease from sinning. Had it been a case of a trespass only, and not of a subsequent corruption, repentance would have been well enough; but when once transgression had begun men came under the power of the corruption proper to their nature and were bereft of the grace which belonged to them as creatures in the Image of God. No, repentance could not meet the case. What—or rather Who was it that was needed for such grace and such recall as we required? Who, save the Word of God Himself, Who also in the beginning had made all things out of nothing? His part it was, and His alone, both to bring again the corruptible to incorruption and to maintain for the Father His consistency of character with all. For He alone, being Word of the Father and above all, was in consequence both able to recreate all, and worthy to suffer on behalf of all and to be an ambassador for all with the Father.
For this purpose, then, the incorporeal and incorruptible and immaterial Word of God entered our world. In one sense, indeed, He was not far from it before, for no part of creation had ever been without Him Who, while ever abiding in union with the Father, yet fills all things that are. But now He entered the world in a new way, stooping to our level in His love and Self-revealing to us. He saw the reasonable race, the race of men that, like Himself, expressed the Father's Mind, wasting out of existence, and death reigning over all in corruption. He saw that corruption held us all the closer, because it was the penalty for the Transgression; He saw, too, how unthinkable it would be for the law to be repealed before it was fulfilled. He saw how unseemly it was that the very things of which He Himself was the Artificer should be disappearing. He saw how the surpassing wickedness of men was mounting up against them; He saw also their universal liability to death. All this He saw and, pitying our race, moved with compassion for our limitation, unable to endure that death should have the mastery, rather than that His creatures should perish and the work of His Father for us men come to nought, He took to Himself a body, a human body even as our own. Nor did He will merely to become embodied or merely to appear; had that been so, He could have revealed His divine majesty in some other and better way. No, He took our body, and not only so, but He took it directly from a spotless, stainless virgin, without the agency of human father—a pure body, untainted by intercourse with man. He, the Mighty One, the Artificer of all, Himself prepared this body in the virgin as a temple for Himself, and took it for His very own, as the instrument through which He was known and in which He dwelt. Thus, taking a body like our own, because all our bodies were liable to the corruption of death, He surrendered His body to death instead of all, and offered it to the Father. This He did out of sheer love for us, so that in His death all might die, and the law of death thereby be abolished because, having fulfilled in His body that for which it was appointed, it was thereafter voided of its power for men. This He did that He might turn again to incorruption men who had turned back to corruption, and make them alive through death by the appropriation of His body and by the grace of His resurrection. Thus He would make death to disappear from them as utterly as straw from fire.
The Word perceived that corruption could not be got rid of otherwise than through death; yet He Himself, as the Word, being immortal and the Father's Son, was such as could not die. For this reason, therefore, He assumed a body capable of death, in order that it, through belonging to the Word Who is above all, might become in dying a sufficient exchange for all, and, itself remaining incorruptible through His indwelling, might thereafter put an end to corruption for all others as well, by the grace of the resurrection. It was by surrendering to death the body which He had taken, as an offering and sacrifice free from every stain, that He forthwith abolished death for His human brethren by the offering of the equivalent. For naturally, since the Word of God was above all, when He offered His own temple and bodily instrument as a substitute for the life of all, He fulfilled in death all that was required. Naturally also, through this union of the immortal Son of God with our human nature, all men were clothed with incorruption in the promise of the resurrection. For the solidarity of mankind is such that, by virtue of the Word's indwelling in a single human body, the corruption which goes with death has lost its power over all. St. Athanasius