Objection 1. It would seem that this is false: "God is man." For every affirmative proposition of remote matter is false. Now this proposition, "God is man," is on remote matter, since the forms signified by the subject and predicate are most widely apart. Therefore, since the aforesaid proposition is affirmative, it would seem to be false.
Objection 2. Further, the three Divine Persons are in greater mutual agreement than the human nature and the Divine. But in the mystery of the Incarnation one Person is not predicated of another; for we do not say that the Father is the Son, or conversely. Therefore it seems that the human nature ought not to be predicated of God by saying that God is man.
Objection 3. Further, Athanasius says (Symb. Fid.) that, "as the soul and the flesh are one man, so are God and man one Christ." But this is false: "The soul is the body." Therefore this also is false: "God is man."
Objection 4. Further, it was said in I, 39, 4 that what is predicated of God not relatively but absolutely, belongs to the whole Trinity and to each of the Persons. But this word "man" is not relative, but absolute. Hence, if it is predicated of God, it would follow that the whole Trinity and each of the Persons is man; and this is clearly false.
On the contrary, It is written (Philippians 2:6-7): "Who being in the form of God... emptied Himself, taking the form of a servant, being made in the likeness of man, and in habit found as a man"; and thus He Who is in the form of God is man. Now He Who is in the form of God is God. Therefore God is man.
I answer that, This proposition "God is man," is admitted by all Christians, yet not in the same way by all. For some admit the proposition, but not in the proper acceptation of the terms. Thus the Manicheans say the Word of God is man, not indeed true, but fictitious man, inasmuch as they say that the Son of God assumed an imaginary body, and thus God is called man as a bronze figure is called man if it has the figure of a man. So, too, those who held that Christ's body and soul were not united, could not say that God is true man, but that He is figuratively called man by reason of the parts. Now both these opinions were disproved above (2, 5; 5, 1).
Some, on the contrary, hold the reality on the part of man, but deny the reality on the part of God. For they say that Christ, Who is God and man, is God not naturally, but by participation, i.e. by grace; even as all other holy men are called gods--Christ being more excellently so than the rest, on account of His more abundant grace. And thus, when it is said that "God is man," God does not stand for the true and natural God. And this is the heresy of Photinus, which was disproved above (2, 10,11). But some admit this proposition, together with the reality of both terms, holding that Christ is true God and true man; yet they do not preserve the truth of the predication. For they say that man is predicated of God by reason of a certain conjunction either of dignity, or of authority, or of affection or indwelling. It was thus that Nestorius held God to be man--nothing further being meant than that God is joined to man by such a conjunction that man is dwelt in by God, and united to Him in affection, and in a share of the Divine authority and honor. And into the same error fall those who suppose two supposita or hypostases in Christ, since it is impossible to understand how, of two things distinct in suppositum or hypostasis, one can be properly predicated of the other: unless merely by a figurative expression, inasmuch as they are united in something, as if we were to say that Peter is John because they are somehow mutually joined together. And these opinions also were disproved above (2, 3,6).
Hence, supposing the truth of the Catholic belief, that the true Divine Nature is united with true human nature not only in person, but also in suppositum or hypostasis; we say that this proposition is true and proper, "God is man"--not only by the truth of its terms, i.e. because Christ is true God and true man, but by the truth of the predication. For a word signifying the common nature in the concrete may stand for all contained in the common nature, as this word "man" may stand for any individual man. And thus this word "God," from its very mode of signification, may stand for the Person of the Son of God, as was said in I, 39, 4. Now of every suppositum of any nature we may truly and properly predicate a word signifying that nature in the concrete, as "man" may properly and truly be predicated of Socrates and Plato. Hence, since the Person of the Son of God for Whom this word "God" stands, is a suppositum of human nature this word man may be truly and properly predicated of this word "God," as it stands for the Person of the Son of God.
Reply to Objection 1. When different forms cannot come together in one suppositum, the proposition is necessarily in remote matter, the subject signifying one form and the predicate another. But when two forms can come together in one suppositum, the matter is not remote, but natural or contingent, as when I say: "Something white is musical." Now the Divine and human natures, although most widely apart, nevertheless come together by the mystery of Incarnation in one suppositum, in which neither exists accidentally, but [both] essentially. Hence this proposition is neither in remote nor in contingent, but in natural matter; and man is not predicated of God accidentally, but essentially, as being predicated of its hypostasis--not, indeed, by reason of the form signified by this word "God," but by reason of the suppositum, which is a hypostasis of human nature.
Reply to Objection 2. The three Divine Persons agree in one Nature, and are distinguished in suppositum; and hence they are not predicated one of another. But in the mystery of Incarnation the natures, being distinct, are not predicated one of the other, in the abstract. For the Divine Nature is not the human nature. But because they agree in suppositum, they are predicated of each other in the concrete.
Reply to Objection 3. "Soul" and "flesh" are taken in the abstract, even as Godhead and manhood; but in the concrete we say "animate" and "carnal" or "corporeal," as, on the other hand, "God" and "man." Hence in both cases the abstract is not predicated of the abstract, but only the concrete of the concrete.
Reply to Objection 4. This word "man" is predicated of God, because of the union in person, and this union implies a relation. Hence it does not follow the rule of those words which are absolutely predicated of God from eternity.
Source: Summa Theologica (New Advent)