Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Commonitory of Vincent of Lérins, For the Antiquity and Universality of the Catholic Faith Against the Profane Novelties of All Heresies.: Chapter XV. The Union of the Divine with the Human Nature took place in the very Conception of the Virgin. The appellation “The Mother of God.”

Chapter XV.

The Union of the Divine with the Human Nature took place in the very Conception of the Virgin. The appellation “The Mother of God.”

[40.] This unity of Person, then, in Christ was not effected after His birth of the Virgin, but was compacted and perfected in her very womb. For we must take most especial heed that we confess Christ not only one, but always one. For it were intolerable blasphemy, if while thou dost confess Him one now, thou shouldst maintain that once He was not one, but two; one forsooth since His baptism, but two at His birth. Which monstrous sacrilege we shall assuredly in no wise avoid unless we acknowledge the manhood united to the Godhead (but by unity of Person), not from the ascension, or the resurrection, or the baptism, but even in His mother, even in the womb, even in the Virgin’s very conception. 474 In consequence of which unity of Person, both those attributes which are proper to God are ascribed to man, and those which are proper to the flesh to God, indifferently and promiscuously. 475 For hence it is written by divine guidance, on the one hand, that the Son of man came down from heaven; 476 and on the other, that the Lord of glory was crucified on earth. 477 Hence it is also that since the Lord’s flesh was made, since the Lord’s flesh was created, the very Word of God is said to have been made, the very omniscient Wisdom of God to have been created, just as prophetically His hands and His feet are described as having been pierced. 478 From this unity of Person it follows, by reason of a like mystery, that, since the flesh of the Word was born of an undefiled mother, God the Word Himself is most Catholicly believed, most impiously denied, to have been born of the Virgin; which being the case, God forbid that any one should seek to defraud Holy Mary of her prerogative of divine grace and her special glory. For by the singular gift of Him who is our Lord and God, and withal, her own son, she is to be confessed most truly and most blessedly—The mother of God p. 143 “Theotocos,” but not in the sense in which it is imagined by a certain impious heresy which maintains, that she is to be called the Mother of God for no other reason than because she gave birth to that man who afterwards became God, just as we speak of a woman as the mother of a priest, or the mother of a bishop, meaning that she was such, not by giving birth to one already a priest or a bishop, but by giving birth to one who afterwards became a priest or a bishop. Not thus, I say, was the holy Mary “Theotocos,” the mother of God, but rather, as was said before, because in her sacred womb was wrought that most sacred mystery whereby, on account of the singular and unique unity of Person, as the Word in flesh is flesh, so Man in God is God. 479

Footnotes

If the Son of God had taken to Himself a man now made and already perfected, it would of necessity follow that there are in Christ two persons, the one assuming and the other assumed; whereas, the Son of God did not assume a man’s person unto His own, but a man’s nature to His own person, and therefore took semen, the seed of Abraham, the very first original element of our nature, before it was come to have any personal human subsistence. The flesh, and the conjunction of the flesh with God, began both in one instant. His making and taking to Himself our flesh was but one act, so that in Christ there is no personal subsistence but one, and that from everlasting. By taking only the nature of man He still continueth one person, and changeth but the manner of His subsisting, which was before in the mere glory of the Son of God and is now in the habit of our flesh.—Hooker, Eccl. Pol. v. 52, § 3.

“A kind of mutual commutation there is, whereby those concrete names, God and man, when we speak of Christ, do take interchangeably one another’s room, so that for truth of speech, it skilleth not, whether we say that the Son of God hath created the world, and the Son of man by His death hath saved it, or else, that the Son of man did create, and the Son of God die to save the world. Howbeit, as oft as we attribute to God what the manhood of Christ claimeth, or to man what His Deity hath right unto, we understand by the name of God and the name of man neither the one nor the other nature, but the whole person of Christ, in whom both natures are.”—Hooker, Eccl. Polity, v. 53, § 4. This is technically called “The Communication of Properties,” Communicatio idiomatum.

St. John iii. 13.

1 Cor. ii. 8.

Ps. xxii. 16.

Sicut Verbum in carne caro, ita Homo in Deo Deus est. Compare the Athanasian Creed, v. 33, in what is probably the true reading, “Unus autem, non conversione Divinitatis in carne, sed assumptione Humanitatis in Deo.”