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 (Q. 2, AA. 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 (Q. 2, AA. 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 the First Part (Q. 39, A. 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 Obj. 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 the 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 Obj. 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 the 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 Obj. 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 Obj. 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. _______________________
SECOND ARTICLE [III, Q. 16, Art. 2]
Whether This Is True: "Man Is God"?
Objection 1: It would seem that this is false: "Man is God." For God is an incommunicable name; hence (Wis. 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.
Obj. 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.