Objection 1. It would seem that this is false: "Man is God." For God is an incommunicable name; hence (Wisdom 13:10; 14:21) idolaters are rebuked for giving the name of God, which is incommunicable, to wood and stones. Hence with equal reason does it seem unbecoming that this word "God" should be predicated of man.
Objection 2. Further, whatever is predicated of the predicate may be predicated of the subject. But this is true: "God is the Father," or "God is the Trinity." Therefore, if it is true that "Man is God," it seems that this also is true: "Man is the Father," or "Man is the Trinity." But these are false. Therefore the first is false.
Objection 3. Further, it is written (Psalm 80:10): "There shall be no new God in thee." But man is something new; for Christ was not always man. Therefore this is false: "Man is God."
On the contrary, It is written (Romans 9:5): "Of whom is Christ according to the flesh, Who is over all things, God blessed for ever." Now Christ, according to the flesh, is man. Therefore this is true: "Man is God."
I answer that, Granted the reality of both natures, i.e. Divine and human, and of the union in person and hypostasis, this is true and proper: "Man is God," even as this: "God is man." For this word "man" may stand for any hypostasis of human nature; and thus it may stand for the Person of the Son of God, Whom we say is a hypostasis of human nature. Now it is manifest that the word "God" is truly and properly predicated of the Person of the Son of God, as was said in I, 39, 4. Hence it remains that this is true and proper: "Man is God."
Reply to Objection 1. Idolaters attributed the name of the Deity to stones and wood, considered in their own nature, because they thought there was something divine in them. But we do not attribute the name of the Deity to the man in His human nature, but in the eternal suppositum, which by union is a suppositum of human nature, as stated above.
Reply to Objection 2. This word "Father" is predicated of this word "God," inasmuch as this word "God" stands for the Person of the Father. And in this way it is not predicated of the Person of the Son, because the Person of the Son is not the Person of the Father. And, consequently, it is not necessary that this word "Father" be predicated of this word "Man," of which the Word "God" is predicated, inasmuch as "Man" stands for the Person of the Son.
Reply to Objection 3. Although the human nature in Christ is something new, yet the suppositum of the human nature is not new, but eternal. And because this word "God" is predicated of man not on account of the human nature, but by reason of the suppositum, it does not follow that we assert a new God. But this would follow, if we held that "Man" stands for a created suppositum: even as must be said by those who assert that there are two supposita in Christ [Cf. 02, 3,6].
Source: Summa Theologica (New Advent)