1. The Son of God was made man because to be man is truly predicated of the Son of God, not from eternity, but from the time of His assuming human nature, the moment of His conception ca. 2000 years ago. “Made man” does not imply a change in God, which would contradict Mal 3:6 (“I am the Lord, and I change not”). Whatever is predicated relatively can be newly predicated of Jack without Jack’s change. The change was only on the part of the human nature assumed and not on the part of God. To illustrate, take the men Jack and John. To be on the right side is predicated of Jack without Jack’s change, for Jack was immobile while John changed by moving to Jack’s left side.
2. The Son of God was made man. Therefore, man is God. For “man” may represent any hypostasis of human nature and thus may represent the Person of the Son of God (a hypostasis of human nature), of whom the word God is truly and properly predicated. 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 the human nature (n.b. there is only one suppositum of both natures and only one hypostasis of one person, contra the heretic Nestorius and his followers; if Christ as man is a hypostasis or person there would be two persons in Christ, one temporal and the other eternal, which is repugnant to the truth of the Incarnation).
3. The man Christ did not begin to be; He always was. Christ began to be Man, since “Man” signifies the eternal, uncreated suppositum when placed in the subject and refers to the finite human nature when placed in the predicate.
4. The characteristics of the Son of Man may be predicated of the Son of God and the characteristics of the Son of God may be predicated of the Son of Man. That means that we can say of Christ that “God is passible,” “the God of glory was crucified,” and “God died.”
5. Because there is one hypostasis of the Divine and the human nature, the name of either nature (Son of Man, Son of God) signifies the same hypostasis. What belongs to the Divine nature be said of the Man, as of a hypostasis of the Divine nature. Moreover, what belongs to the human nature may be said of God, as of a hypostasis of human nature. The Catholic faithful do not distinguish things predicated of Christ but distinguish the reasons for which they are predicated.
6. Now the critical thinker will not fall into the trap of predicating the characteristics of the Son of Man of the Divine nature and the characteristics of the Son of God of the human nature. In the Mystery of the Incarnation the Divine and human natures are not the same, though the hypostasis of the two natures in the same. Therefore what belongs to one nature cannot be predicated of the other if they are taken in the abstract. Now concrete words stand for the hypostasis of the nature. Ergo of concrete words we may predicate simply what belongs to either nature, whether the concrete word of which they are predicated refers to the Divine nature alone or the human nature alone.
7. Therefore we affirm that “God is passible” but we deny that “the Godhead is passible.”
Source: ST 3.16.2,4-6,9,12