Objection 1. It would seem that the work of Incarnation ought to have been put off till the end of the world. For it is written (Psalm 91:11): "My old age in plentiful mercy"--i.e. "in the last days," as a gloss says. But the time of Incarnation is especially the time of mercy, according to Psalm 101:14: "For it is time to have mercy on it." Therefore the Incarnation ought to have been put off till the end of the world.
Objection 2. Further, as has been said (5, ad 3), in the same subject, perfection is subsequent in time to imperfection. Therefore, what is most perfect ought to be the very last in time. But the highest perfection of human nature is in the union with the Word, because "in Christ it hath pleased the Father that all the fulness of the Godhead should dwell," as the Apostle says (Colossians 1:19, and 2:9). Therefore Incarnation ought to have been put off till the end of the world.
Objection 3. Further, what can be done by one ought not to be done by two. But the one coming of Christ at the end of the world was sufficient for the salvation of human nature. Therefore it was not necessary for Him to come beforehand in His Incarnation; and hence Incarnation ought to have been put off till the end of the world.
On the contrary, It is written (Habakkuk 3:2): "In the midst of the years Thou shalt make it known." Therefore the mystery of Incarnation which was made known to the world ought not to have been put off till the end of the world.
I answer that, As it was not fitting that God should become incarnate at the beginning of the world, so also it was not fitting that the Incarnation should be put off till the end of the world. And this is shown first from the union of the Divine and human nature. For, as it has been said (5, ad 3), perfection precedes imperfection in time in one way, and contrariwise in another way imperfection precedes perfection. For in that which is made perfect from being imperfect, imperfection precedes perfection in time, whereas in that which is the efficient cause of perfection, perfection precedes imperfection in time. Now in the work of Incarnation both concur; for by Incarnation human nature is raised to its highest perfection; and in this way it was not becoming that Incarnation should take place at the beginning of the human race. And the Word incarnate is the efficient cause of the perfection of human nature, according to John 1:16: "Of His fulness we have all received"; and hence the work of Incarnation ought not to have been put off till the end of the world. But the perfection of glory to which human nature is to be finally raised by the Word Incarnate will be at the end of the world.
Secondly, from the effect of man's salvation; for, as is said Qq. Vet et Nov. Test., qu. 83, "it is in the power of the Giver to have pity when, or as much as, He wills. Hence He came when He knew it was fitting to succor, and when His boons would be welcome. For when by the feebleness of the human race men's knowledge of God began to grow dim and their morals lax, He was pleased to choose Abraham as a standard of the restored knowledge of God and of holy living; and later on when reverence grew weaker, He gave the law to Moses in writing; and because the gentiles despised it and would not take it upon themselves, and they who received it would not keep it, being touched with pity, God sent His Son, to grant to all remission of their sin and to offer them, justified, to God the Father." But if this remedy had been put off till the end of the world, all knowledge and reverence of God and all uprightness of morals would have been swept away from the earth.
Thirdly, this appears fitting to the manifestation of the Divine power, which has saved men in several ways--not only by faith in some future thing, but also by faith in something present and past.
Reply to Objection 1. This gloss has in view the mercy of God, which leads us to glory. Nevertheless, if it is referred to the mercy shown the human race by Incarnation of Christ, we must reflect that, as Augustine says (Retract. i), the time of Incarnation may be compared to the youth of the human race, "on account of the strength and fervor of faith, which works by charity"; and to old age--i.e. the sixth age--on account of the number of centuries, for Christ came in the sixth age. And although youth and old age cannot be together in a body, yet they can be together in a soul, the former on account of quickness, the latter on account of gravity. And hence Augustine says elsewhere (Qq. lxxxiii, qu. 44) that "it was not becoming that the Master by Whose imitation the human race was to be formed to the highest virtue should come from heaven, save in the time of youth." But in another work (De Gen. cont. Manich. i, 23) he says: that Christ came in the sixth age--i.e. in the old age--of the human race.
Reply to Objection 2. The work of Incarnation is to be viewed not as merely the terminus of a movement from imperfection to perfection, but also as a principle of perfection to human nature, as has been said.
Reply to Objection 3. As Chrysostom says on John 3:11, "For God sent not His Son into the world to judge the world" (Hom. xxviii): "There are two comings of Christ: the first, for the remission of sins; the second, to judge the world. For if He had not done so, all would have perished together, since all have sinned and need the glory of God." Hence it is plain that He ought not to have put off the coming in mercy till the end of the world.
Source: Summa Theologica (New Advent)