Objection 1. It would seem that the justification of the ungodly is not God's greatest work. For it is by the justification of the ungodly that we attain the grace of a wayfarer. Now by glorification we receive heavenly grace, which is greater. Hence the glorification of angels and men is a greater work than the justification of the ungodly.
Objection 2. Further, the justification of the ungodly is ordained to the particular good of one man. But the good of the universe is greater than the good of one man, as is plain from Ethic. i, 2. Hence the creation of heaven and earth is a greater work than the justification of the ungodly.
Objection 3. Further, to make something from nothing, where there is nought to cooperate with the agent, is greater than to make something with the cooperation of the recipient. Now in the work of creation something is made from nothing, and hence nothing can cooperate with the agent; but in the justification of the ungodly God makes something from something, i.e. a just man from a sinner, and there is a cooperation on man's part, since there is a movement of the free-will, as stated above (Article 3). Hence the justification of the ungodly is not God's greatest work.
On the contrary, It is written (Psalm 144:9): "His tender mercies are over all His works," and in a collect [Tenth Sunday after Pentecost] we say: "O God, Who dost show forth Thine all-mightiness most by pardoning and having mercy," and Augustine, expounding the words, "greater than these shall he do" (John 14:12) says that "for a just man to be made from a sinner, is greater than to create heaven and earth."
I answer that, A work may be called great in two ways: first, on the part of the mode of action, and thus the work of creation is the greatest work, wherein something is made from nothing; secondly, a work may be called great on account of what is made, and thus the justification of the ungodly, which terminates at the eternal good of a share in the Godhead, is greater than the creation of heaven and earth, which terminates at the good of mutable nature. Hence, Augustine, after saying that "for a just man to be made from a sinner is greater than to create heaven and earth," adds, "for heaven and earth shall pass away, but the justification of the ungodly shall endure."
Again, we must bear in mind that a thing is called great in two ways: first, in an absolute quantity, and thus the gift of glory is greater than the gift of grace that sanctifies the ungodly; and in this respect the glorification of the just is greater than the justification of the ungodly. Secondly, a thing may be said to be great in proportionate quantity, and thus the gift of grace that justifies the ungodly is greater than the gift of glory that beatifies the just, for the gift of grace exceeds the worthiness of the ungodly, who are worthy of punishment, more than the gift of glory exceeds the worthiness of the just, who by the fact of their justification are worthy of glory. Hence Augustine says: "Let him that can, judge whether it is greater to create the angels just, than to justify the ungodly. Certainly, if they both betoken equal power, one betokens greater mercy."
And thus the reply to the first is clear.
Reply to Objection 2. The good of the universe is greater than the particular good of one, if we consider both in the same genus. But the good of grace in one is greater than the good of nature in the whole universe.
Reply to Objection 3. This objection rests on the manner of acting, in which way creation is God's greatest work.
Source: Summa Theologica (New Advent)