Thomas Aquinas, Compendium of Theology

Nestorius wished to avoid this contradiction. In part he disagreed with the error of Photinus; for Nestorius held that Christ was the Son of God not only by the grace of adoption, but by the divine nature in which He existed coeternal with the Father. In part, however, he sided with Photinus, because he taught that the Son of God was united to man by mere habitation in him, but not in such a way that there was only one person who was both God and man. And so that man who, according to Photinus, is called God through grace alone, is called the son of God by Nestorius, not because he is truly God, but because the Son of God dwells in him through the inhabitation effected by grace.

This error is likewise opposed to the authority of Sacred Scripture. For the union of God with man is called by the Apostle an "emptying"; in Philippians 2:6 he says of the Son of God: "Who, being in the form of God, thought is not robbery to be equal with God, but emptied Himself, taking the form of a servant." But there is no emptying of God when He dwells in a rational creature by grace. Otherwise the Father and the Holy Spirit would be emptied out also, since they too dwell in the rational creature by grace. Thus in John 14:23 our Lord says of Himself and the Father: "We will come to him and will make Our abode with him." And in I Corinthians 3:16 the Apostle says of the Holy Spirit: "The Spirit of God dwelleth in you."

Moreover, the man in question could hardly use words signifying divinity unless He were personally God. He would have been guilty of supreme presumption in saying, as He does in John 10:30: "I and the Father are one," and also in 8:58: "Before Abraham was made, I am." For the pronoun "I" indicates the person of the speaker; but He who uttered these words was a man. Hence the person of God and this man are one and the same.

To preclude such errors, both the Apostles' Creed and the Creed of the Nicene Fathers, after mentioning the person of the Son, add that He was conceived of the Holy Ghost, was born, suffered, died, and rose. Surely what pertains to the man would not be predicated of the Son of God unless the person of the Son of God and of the man were the same. What is proper to one person is, by that very fact, not said of another person; for example, what is proper to Paul is, for that precise reason, not predicated of Peter.