7 Therefore, as I have already said, He caused man (human nature) to cleave to and to become, one with God. For unless man had overcome the enemy of man, the enemy would not have been legitimately vanquished. And again: unless it had been God who had freely given salvation, we could never have possessed it securely. And unless man had been joined to God, he could never have become a partaker of incorruptibility. For it was incumbent upon the Mediator between God and men, by His relationship to both, to bring both to friendship and concord, and present man to God, while He revealed God to man. For, in what way could we be partaken of the adoption of sons, unless we had received from Him through the Son that fellowship which refers to Himself, unless His Word, having been made flesh, had entered into communion with us?
Wherefore also He passed through every stage of life, restoring to all communion with God. Those, therefore, who assert that He appeared putatively, and was neither born in the flesh nor truly made man, are as yet under the old condemnation, holding out patronage to sin; for, by their showing, death has not been vanquished, which “reigned from Adam to Moses, even over them that had not sinned after the similitude of Adam's transgression.” But the law coming, which was given by Moses, and testifying of sin that it is a sinner, did truly take away his (death's) kingdom, showing that he was no king, but a robber; and it revealed him as a murderer.
It laid, however, a weighty burden upon man, who had sin in himself, showing that he was liable to death. For as the law was spiritual, it merely made sin to stand out in relief, but did not destroy it. For sin had no dominion over the spirit, but over man. For it behooved Him who was to destroy sin, and redeem man under the power of death, that He should Himself be made that very same thing which he was, that is, man; who had been drawn by sin into bondage, but was held by death, so that sin should be destroyed by man, and man should go forth from death.
For as by the disobedience of the one man who was originally moulded from virgin soil, the many were made sinners, and forfeited life; so was it necessary that, by the obedience of one man, who was originally born from a virgin, many should be justified and receive salvation. Thus, then, was the Word of God made man, as also Moses says: “God, true are His works.” But if, not having been made flesh, He did appear as if flesh, His work was not a true one. But what He did appear, that He also was: God recapitulated in Himself the ancient formation of man, that He might kill sin, deprive death of its power, and vivify man; and therefore His works are true.
Source: Adversus haereses (New Advent)