Objection 1. It would seem that, after Penance, man rises again to equal virtue. For the Apostle says (Romans 8:28): "To them that love God all things work together unto good," whereupon a gloss of Augustine says that "this is so true that, if any such man goes astray and wanders from the path, God makes even this conduce to his good." But this would not be true if he rose again to lesser virtue. Therefore it seems that a penitent never rises again to lesser virtue.
Objection 2. Further, Ambrose says [Cf. Hypognosticon iii, an anonymous work falsely ascribed to St. Augustine] that "Penance is a very good thing, for it restores every defect to a state of perfection." But this would not be true unless virtues were recovered in equal measure. Therefore equal virtue is always recovered through Penance.
Objection 3. Further, on Genesis 1:5: "There was evening and morning, one day," a gloss says: "The evening light is that from which we fall the morning light is that to which we rise again." Now the morning light is greater than the evening light. Therefore a man rises to greater grace or charity than that which he had before; which is confirmed by the Apostle's words (Romans 5:20): "Where sin abounded, grace did more abound."
On the contrary, Charity whether proficient or perfect is greater than incipient charity. But sometimes a man falls from proficient charity, and rises again to incipient charity. Therefore man always rises again to less virtue.
I answer that, As stated above (86, 6, ad 3; 89, 1, ad 2), the movement of the free-will, in the justification of the ungodly, is the ultimate disposition to grace; so that in the same instant there is infusion of grace together with the aforesaid movement of the free-will, as stated in I-II, 113, 5,7, which movement includes an act of penance, as stated above (Question 86, Article 2). But it is evident that forms which admit of being more or less, become intense or remiss, according to the different dispositions of the subject, as stated in I-II, 52, 1,2; I-II, 66, 1]. Hence it is that, in Penance, according to the degree of intensity or remissness in the movement of the free-will, the penitent receives greater or lesser grace. Now the intensity of the penitent's movement may be proportionate sometimes to a greater grace than that from which man fell by sinning, sometimes to an equal grace, sometimes to a lesser. Wherefore the penitent sometimes arises to a greater grace than that which he had before, sometimes to an equal, sometimes to a lesser grace: and the same applies to the virtues, which flow from grace.
Reply to Objection 1. The very fact of falling away from the love of God by sin, does not work unto the good of all those who love God, which is evident in the case of those who fall and never rise again, or who rise and fall yet again; but only to the good of "such as according to His purpose are called to be saints," viz. the predestined, who, however often they may fall, yet rise again finally. Consequently good comes of their falling, not that they always rise again to greater grace, but that they rise to more abiding grace, not indeed on the part of grace itself, because the greater the grace, the more abiding it is, but on the part of man, who, the more careful and humble he is, abides the more steadfastly in grace. Hence the same gloss adds that "their fall conduces to their good, because they rise more humble and more enlightened."
Reply to Objection 2. Penance, considered in itself, has the power to bring all defects back to perfection, and even to advance man to a higher state; but this is sometimes hindered on the part of man, whose movement towards God and in detestation of sin is too remiss, just as in Baptism adults receive a greater or a lesser grace, according to the various ways in which they prepare themselves.
Reply to Objection 3. This comparison of the two graces to the evening and morning light is made on account of a likeness of order, since the darkness of night follows after the evening light, and the light of day after the light of morning, but not on account of a likeness of greater or lesser quantity. Again, this saying of the Apostle refers to the grace of Christ, which abounds more than any number of man's sins. Nor is it true of all, that the more their sins abound, the more abundant grace they receive, if we measure habitual grace by the quantity. Grace is, however, more abundant, as regards the very notion of grace, because to him who sins more a more "gratuitous" favor is vouchsafed by his pardon; although sometimes those whose sins abound, abound also in sorrow, so that they receive a more abundant habit of grace and virtue, as was the case with Magdalen.
To the argument advanced in the contrary sense it must be replied that in one and the same man proficient grace is greater than incipient grace, but this is not necessarily the case in different men, for one begins with a greater grace than another has in the state of proficiency: thus Gregory says (Dial. ii, 1): "Let all, both now and hereafter, acknowledge how perfectly the boy Benedict turned to the life of grace from the very beginning."
Source: Summa Theologica (New Advent)