"It behooved Christ to rise again ... for our instruction in the faith, since our belief in Christ's Godhead is confirmed by His rising again." (Summa III.53.1)
"The Resurrection of the crucified one shows that he was truly "I AM", the Son of God and God himself." (Catechism, 653)
It seems that it had to be that way. He had to be touched by Mary Magdalene, to eat fish with the apostles, to break bread with the disciples on the road to Emmaus, to cook dinner on the shores of the sea of Tiberias. Thomas the Doubter had to "put his finger into his hands ... his hand into his side" in order that we all might believe. It was necessary that Christ should rise incarnate, in the flesh, in the same body in which he had lived for thirty-three years. Had he chosen to leave that body behind, he would no longer have been one of us, and we could not have been so sure of his divinity.
St. Thomas comes near to asking this question (see Summa III, Questions 57, "the ascension of Christ"; 58, "sitting at the right hand of the Father"); and 59, "Christ's judiciary power"), but never quite does ask it. So I think it would be fitting (conveniens) to put the problem as St. Thomas might have put it. Behold! the apocryphal Article 7 of Question 59 of the third part of the Summa Theologiae of St. Thomas Aquinas.
Whether the Son will retain his human nature after the Last Judgment.
Objection 1. It seems that the Son of God ought not to retain his human nature after the Last Judgment. For it was necessary that he should rise as a man for our instruction in faith, as has been said (Summa III.53.1), and it was likewise necessary that he ascend in the flesh, and in the flesh make intercession for us (Hebrews 9:24), and judge us as a man (as has also been proven, Summa III.59.2). But it would seem to be superfluous after these things, that he should retain his humanity; and in God there is no superfluity, because he is a necessary Being. Therefore, it seems that the Son will not to retain his human nature after the Last Judgment.
Objection 2. Furthermore, as has been objected elsewhere in this work, "it is not fitting to unite things that are infinitely apart," such as God and man; and if, though not absolutely fitting, it was so for our salvation, it seems that once the necessity for salvation has passed, it would be more fitting that the union should cease. Therefore, it seems that the Son will not retain his human nature after the Last Judgment.
Objection 3. Furthermore, it is acknowledged by the faithful that all things exist for the glory of God. But that God should remain as man in eternity seems rather to give glory to human beings; therefore, it seems that the Son will not retain his human nature after the Last Judgment.
On the contrary, when we pray the creed, we say that "He is seated at the right hand of the Father." And St. John Damascene writes in explanation (Defide orth. 4:2) that "we understand the right hand of the Father to be the glory and honour of the Godhead in which the Son of God, who existed as God before the ages, and is of like essence to the Father, and in the end became flesh, has a seat in the body, His flesh sharing in the glory. For He along with His flesh is adored with one adoration by all creation."
I answer that, it is not perhaps necessary, in the strictest sense of the word, for the Son of God to retain his humanity after the Last Judgment, but it is clear from Scripture that he will do so. Daniel writes (7:13-14) that the Father ("the Ancient of days") gives Christ ("one like the son of man") "power, and glory, and a kingdom: and all peoples, tribes and tongues shall serve him: his power is an everlasting power that shall not be taken away: and his kingdom that shall not be destroyed." Again, St. Paul writes (Philippians 2:9-10) that "God also hath exalted [Christ], and hath given him a name which is above all names: That in the name of Jesus every knee should bow, of those that are in heaven, on earth, and under the earth." Now in these passages, it is to Christ Jesus that the power and the glory and the kingdom are given: for the authors say "one like the son of man" and "Jesus," thereby indicating his humanity; and these things are furthermore given to him for all times ("everlasting") and in all places ("in heaven, on earth, and under the earth"), indicating the permanence and universality of his reign.
Reply to Objection 1. The one making the objection is mistaken in supposing that the only alternative to strict necessity is superfluity; for in fact many things are neither strictly necessary nor superfluous but either contingently necessary or proper or something of that sort. Furthermore, the one making the objection seems to suppose that "necessary being" means "one who can act only out of necessity." If that were true, God could not have created anything, much less become incarnate, which is patently false. To put it plainly, the fact that God "must" exist does not mean that he must do or not do certain things; on the contrary, it means that his freedom to do and not do is unlimited.
Reply to Objection 2. The reply to the original objection argued that it was fitting for man's salvation for human flesh to be united to God, although purely in itself the union would have been unfitting, because of the disparity of dignity. And just as God desired to save man, so that the Incarnation was fitting for that purpose, even so God has desired that man should participate in God's divinity, making it fitting for that purpose for his Son to retain human nature even after the Judgment. Hence it is written (1 John 3:2) "we shall be like to him."
Reply to Objection 3. It is right to insist that all things exist for the glory of God. But those who understand that glory to be opposed to God's love for and condescension towards the human race are thinking not as God does but as men do—as if God had only so much glory, and by giving some to men, he diminished himself; when in fact the exact opposite is true. For God came not only to free us, but to raise us to himself; and so we are called "sons of God," and again, "not servants, but friends." As St. Augustine puts it (xiii de Temp.), "God was made man, that man might be made God." This "full participation in the Divinity" (Summa III.1.2), which the Incarnation made possible, is wholly beyond any human merit or desert; and it is precisely because of our poverty in this regard that God's great love is made clear, for which cause we give him even greater glory than we would otherwise have done, had his love for us been less manifest to our understanding.
For our love is like ourselves: being small in stature, and apt to fail, it seeks after what is good. But God's Love is like God; rather, God's Love is God, and "God is Love;" and that which Love loves, by the selfsame act He makes good. And just as a human lover, because he wills the good for his beloved, is "placed outside himself," as they say, so too God in willing our good "'is placed outside Himself.'" And the human lover becomes like the beloved accidentally, that is, without intending it, through loving the beloved for the goodness it has which he lacks. But the Divine Love becomes like the beloved knowingly, though in accidental things: because He loves the beloved not for her goodness, but from His own, and in so loving desires to raise the beloved towards Himself, and so stoops, as it were, to meet her, and pours Himself out to fill her emptiness. Nor does Love in so loving abandon His nature—for it is the nature of Love to stoop, and to give everything away—and likewise it is the nature of Love, which is infinite, never to be emptied, to lose nothing by this pouring out of self, but rather to make all that He touches, and all that He fills, also love.
So yes, I guess ... technically, he doesn't have to stay human ...