Objection 1. It would seem unfitting that Christ should be predestinated. For the term of anyone's predestination seems to be the adoption of sons, according to Ephesians 1:5: "Who hath predestinated us unto the adoption of children." But it is not befitting to Christ to be an adopted Son, as stated above (Question 23, Article 4). Therefore it is not fitting that Christ be predestinated.
Objection 2. Further, we may consider two things in Christ: His human nature and His person. But it cannot be said that Christ is predestinated by reason of His human nature; for this proposition is false--"The human nature is Son of God." In like manner neither by reason of the person; for this person is the Son of God, not by grace, but by nature: whereas predestination regards what is of grace, as stated in I, 23, 2,5. Therefore Christ was not predestinated to be the Son of God.
Objection 3. Further, just as that which has been made was not always, so also that which was predestinated; since predestination implies a certain antecedence. But, because Christ was always God and the Son of God, it cannot be said that that Man was "made the Son of God." Therefore, for a like reason, we ought not to say that Christ was "predestinated the Son of God."
On the contrary, The Apostle says, speaking of Christ (Romans 1:4): "Who was predestinated the Son of God in power."
I answer that, As is clear from what has been said in I, 23, 1,2, predestination, in its proper sense, is a certain Divine preordination from eternity of those things which are to be done in time by the grace of God. Now, that man is God, and that God is man, is something done in time by God through the grace of union. Nor can it be said that God has not from eternity pre-ordained to do this in time: since it would follow that something would come anew into the Divine Mind. And we must needs admit that the union itself of natures in the Person of Christ falls under the eternal predestination of God. For this reason do we say that Christ was predestinated.
Reply to Objection 1. The Apostle there speaks of that predestination by which we are predestinated to be adopted sons. And just as Christ in a singular manner above all others is the natural Son of God, so in a singular manner is He predestinated.
Reply to Objection 2. As a gloss [From St. Augustine, De Praed. Sanct. xv] says on Romans 1:4, some understood that predestination to refer to the nature and not to the Person--that is to say, that on human nature was bestowed the grace of being united to the Son of God in unity of Person.
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 to Objection 3. Origen commenting on Romans 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."
Source: Summa Theologica (New Advent)