On The Holy Trinity by St. Augustine
Chapter 18.--Why the Son of God Took Man Upon Himself from the Race of Adam, and from a Virgin.
23. For assuredly God could have taken upon Himself to be man, that in that manhood He might be the Mediator between God and men, from some other source, and not from the race of that Adam who bound the human race by his sin; as He did not create him whom He first created, of the race of some one else. Therefore He was able, either so, or in any other mode that He would, to create yet one other, by whom the conqueror of the first might be conquered. But God judged it better both to take upon Him man through whom to conquer the enemy of the human race, from the race itself that had been conquered; and yet to do this of a virgin, whose conception, not flesh but spirit, not lust but faith, preceded. Nor did that concupiscence of the flesh intervene, by which the rest of men, who derive original sin, are propagated and conceived; but holy virginity became pregnant, not by conjugal intercourse, but by faith, -- lust being utterly absent, -- so that that which was born from the root of the first man might derive only the origin of race, not also of guilt. For there was born, not a nature corrupted by the contagion of transgression, but the one only remedy of all such corruptions. There was born, I say, a Man having nothing at all, and to have nothing at all, of sin; through whom they were to be born again so as to be freed from sin, who could not be born without sin. For although conjugal chastity makes a right use of the carnal concupiscence which is in our members; yet it is liable to motions not voluntary, by which it shows either that it could not have existed at all in paradise before sin, or if it did, that it was not then such as that sometimes it should resist the will. But now we feel it to be such, that in opposition to the law of the mind, and even if there is no question of begetting, it works in us the incitement of sexual intercourse; and if in this men yield to it, then it is satisfied by an act of sin; if they do not, then it is bridled by an act of refusal: which two things who could doubt to have been alien from paradise before sin? For neither did the chastity that then was do anything indecorous, nor did the pleasure that then was suffer anything unquiet. It was necessary, therefore, that this carnal concupiscence should be entirely absent, when the offspring of the Virgin was conceived; in whom the author of death was to find nothing worthy of death, and yet was to slay Him in order that he might be conquered by the death of the Author of life: the conqueror of the first Adam, who held fast the human race, conquered by the second Adam, and losing the Christian race, freed out of the human race from human guilt, through Him who was not in the guilt, although He was of the race; that that deceiver might be conquered by that race which he had conquered by guilt. And this was so done, in order that man may not be lifted up, but |that he that glorieth should glory in the Lord.| For he who was conquered was only man; and he was therefore conquered, because he lusted proudly to be a god. But He who conquered was both man and God; and therefore He so conquered, being born of a virgin, because God in humility did not, as He governs other saints, so govern that Man, but bare Him [as a Son]. These so great gifts of God, and whatever else there are, which it is too long for us now upon this subject both to inquire and to discuss, could not exist unless the Word had been made flesh.