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 the Incarnation of Christ, we must reflect that, as Augustine says (Retract. i), the time of the 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 the 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 Jn. 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.