First of all, we need to show the reason why the Son of God had to be born of a Virgin. The initiator of a new birth had to be born in a new way, and Isaiah had predicted that the Lord would give a sign of this. What is that sign? “Behold, the virgin shall conceive in her womb and bear a Son” (Is 7:14). Therefore the Virgin conceived and bore Emmanuel, God-with-us.
And this is the new birth: that man is born in God when God is born in man, having assumed the flesh of the old seed, but without using this seed, in order to reform the flesh through a new seed, a spiritual seed, and to purify the flesh after having eliminated all its ancient stains. But, as it happened, this whole new manner of birth was prefigured in the ancient wise design that depended upon a virgin. When man was created by God’s action, the earth was still virgin, not yet pressed down by man’s toil, not having been sown. We know that, from this virgin earth, God created man as a living soul.
If, then, the first Adam was introduced in this way, all the more reason that the second Adam, as the apostle said, had to come forth from a virgin earth, that is, from a body not yet violated by generation, by God’s action, so that he might become the spirit who gives life. However, lest my introduction of Adam’s name appear meaningless, why did the apostle call Christ “Adam” (cf. 1 Cor 15:45), if his humanity did not have an earthly origin? But here, too, reason comes to our aid: through a contrary operation, God recovered his image and likeness, which had been stolen by the devil.
For just as the death-creating word of the devil had penetrated Eve, who was still a virgin, analogously the life-building Word of God had to enter into a Virgin, so that he who had fallen into perdition because of a woman might be led back to salvation by means of the same sex. Eve believed the serpent; Mary believed Gabriel. The fault that Eve introduced by believing, Mary, by believing, erased.
—De carne Christi 17, 1-5; PL 2, 827-28, as found in Luigi Gambero’s Mary and the Fathers of the Church: The Blessed Virgin Mary in Patristic Thought, Ignatius, 1999.