Objection 1. It would seem that this is false: "God was made man." For since man signifies a substance, to be made man is to be made simply. But this is false: "God was made simply." Therefore this is false: "God was made man."
Objection 2. Further, to be made man is to be changed. But God cannot be the subject of change, according to Malachi 3:6: "I am the Lord, and I change not." Hence this is false: "God was made man."
Objection 3. Further, man as predicated of Christ stands for the Person of the Son of God. But this is false: "God was made the Person of the Son of God." Therefore this is false: "God was made man."
On the contrary, It is written (John 1:14): "The Word was made flesh": and as Athanasius says (Ep. ad Epictetum), "when he said, 'The Word was made flesh,' it is as if it were said that God was made man."
I answer that, A thing is said to be made that which begins to be predicated of it for the first time. Now to be man is truly predicated of God, as stated above (Article 1), yet in such sort that it pertains to God to be man, not from eternity, but from the time of His assuming human nature. Hence, this is true, "God was made man"; though it is understood differently by some: even as this, "God is man," as we said above (Article 1).
Reply to Objection 1. To be made man is to be made simply, in all those in whom human nature begins to be in a newly created suppositum. But God is said to have been made man, inasmuch as the human nature began to be in an eternally pre-existing suppositum of the Divine Nature. And hence for God to be made man does not mean that God was made simply.
Reply to Objection 2. As stated above, to be made implies that something. is newly predicated of another. Hence, whenever anything is predicated of another, and there is a change in that of which it is predicated, then to be made is to be changed; and this takes place in whatever is predicated absolutely, for whiteness or greatness cannot newly affect anything, unless it be newly changed to whiteness or greatness. But whatever is predicated relatively can be newly predicated of anything without its change, as a man may be made to be on the right side without being changed and merely by the change of him on whose left side he was. Hence in such cases, not all that is said to be made is changed, since it may happen by the change of something else. And it is thus we say of God: "Lord, Thou art made [Douay: 'hast been'] our refuge" (Psalm 89:1). Now to be man belongs to God by reason of the union, which is a relation. And hence to be man is newly predicated of God without any change in Him, by a change in the human nature, which is assumed to a Divine Person. And hence, when it is said, "God was made man," we understand no change on the part of God, but only on the part of the human nature.
Reply to Objection 3. Man stands not for the bare Person of the Son of God, but inasmuch as it subsists in human nature. Hence, although this is false, "God was made the Person of the Son of God," yet this is true: "God was made man" by being united to human nature.
Source: Summa Theologica (New Advent)