But in that case the phrase of the Apostle would be improper, for two reasons. First, for a general reason: for we do not speak of a person's nature, but of his person, as being predestinated: because to be predestinated is to be directed towards salvation, which belongs to a suppositum acting for the end of beatitude. Secondly, for a special reason. Because to be Son of God is not befitting to human nature; for this proposition is false: "The human nature is the Son of God": unless one were to force from it such an exposition as: "Who was predestinated the Son of God in power"—that is, "It was predestinated that the Human nature should be united to the Son of God in the Person."
Hence we must attribute predestination to the Person of Christ: not, indeed, in Himself or as subsisting in the Divine Nature, but as subsisting in the human nature. Wherefore the Apostle, after saying, "Who was made to Him of the seed of David according to the flesh," added, "Who was predestinated the Son of God in power": so as to give us to understand that in respect of His being of the seed of David according to the flesh, He was predestinated the Son of God in power. For although it is natural to that Person, considered in Himself, to be the Son of God in power, yet this is not natural to Him, considered in the human nature, in respect of which this befits Him according to the grace of union.
Reply Obj. 3: Origen commenting on Rom. 1:4 says that the true reading of this passage of the Apostle is: "Who was destined to be the Son of God in power"; so that no antecedence is implied. And so there would be no difficulty. Others refer the antecedence implied in the participle "predestinated," not to the fact of being the Son of God, but to the manifestation thereof, according to the customary way of speaking in Holy Scripture, by which things are said to take place when they are made known; so that the sense would be—"Christ was predestinated to be made known as the Son of God." But this is an improper signification of predestination. For a person is properly said to be predestinated by reason of his being directed to the end of beatitude: but the beatitude of Christ does not depend on our knowledge thereof.
It is therefore better to say that the antecedence implied in the participle "predestinated" is to be referred to the Person not in Himself, but by reason of the human nature: since, although that Person was the Son of God from eternity, it was not always true that one subsisting in human nature was the Son of God. Hence Augustine says (De Praedest. Sanct. xv): "Jesus was predestinated, so that He Who according to the flesh was to be the son of David, should be nevertheless Son of God in power."
Moreover, it must be observed that, although the participle "predestinated," just as this participle "made," implies antecedence, yet there is a difference. For "to be made" belongs to the thing in itself: whereas "to be predestinated" belongs to someone as being in the apprehension of one who pre-ordains. Now that which is the subject of a form or nature in reality, can be apprehended either as under that form or absolutely. And since it cannot be said absolutely of the Person of Christ that He began to be the Son of God, yet this is becoming to Him as understood or apprehended to exist in human nature, because at one time it began to be true that one existing in human nature was the Son of God; therefore this proposition—"Christ was predestinated the Son of God"—is truer than this—"Christ was made the Son of God." _______________________
SECOND ARTICLE [III, Q. 24, Art. 2]
Whether This Proposition Is False: "Christ As Man Was Predestinated to Be the Son of God"?
Objection 1: It would seem that this proposition is false: "Christ as man was predestinated to be the Son of God." For at some time a man is that which he was predestinated to be: since God's predestination does not fail. If, therefore, Christ as man was predestinated the Son of God, it seems to follow that as man He is the Son of God. But the latter is false. Therefore the former is false.
Obj. 2: Further, what is befitting to Christ as man is befitting to any man; since He belongs to the same species as other men. If, therefore, Christ, as man, was predestinated the Son of God, it will follow that this is befitting to any other man. But the latter is false. Therefore the former is false.
Obj. 3: Further, that is predestinated from eternity which is to take place at some time. But this proposition, "The Son of God was made man," is truer than this, "Man was made the Son of God." Therefore this proposition, "Christ, as the Son of God, was predestinated to be man," is truer than this, "Christ as Man was predestinated to be the Son of God."