The Enchiridion

Now of this Mediator it would occupy too much space to say anything at all worthy of Him; and, indeed, to say what is worthy of Him is not in the power of man. For who will explain in consistent words this single statement, that “the Word was made flesh, and dwelt among us,” so that we may believe in the only Son of God the Father Almighty, born of the Holy Ghost and the Virgin Mary. The meaning of the Word being made flesh, is not that the divine nature was changed into flesh, but that the divine nature assumed our flesh.

And by “flesh” we are here to understand “man,” the part being put for the whole, as when it is said: “By the deeds of the law shall no flesh be justified,” that is, no man. For we must believe that no part was wanting in that human nature which He put on, save that it was a nature wholly free from every taint of sin—not such a nature as is conceived between the two sexes through carnal lust, which is born in sin, and whose guilt is washed away in regeneration; but such as it behooved a virgin to bring forth, when the mother's faith, not her lust, was the condition of conception.

And if her virginity had been marred even in bringing Him forth, He would not have been born of a virgin; and it would be false (which God forbid) that He was born of the Virgin Mary, as is believed and declared by the whole Church, which, in imitation of His mother, daily brings forth members of His body, and yet remains a virgin. Read, if you please, my letter on the virginity of the holy Mary which I sent to that eminent man, whose name I mention with respect and affection, Volusianus.