Objection 1. It would seem that penance is unfittingly divided into penance before Baptism, penance for mortal, and penance for venial sins. For Penance is the second plank after shipwreck, as stated above (Question 84, Article 6), while Baptism is the first. Therefore that which precedes Baptism should not be called a species of penance.
Objection 2. Further, that which can destroy the greater, can destroy the lesser. Now mortal sin is greater than venial; and penance which regards mortal sins regards also venial sins. Therefore they should not be considered as different species of penance.
Objection 3. Further, just as after Baptism man commits venial and mortal sins, so does he before Baptism. If therefore penance for venial sins is distinct from penance for mortal sins after Baptism, in like manner they should be distinguished before Baptism. Therefore penance is not fittingly divided into these species.
On the contrary, Augustine says in De Poenitentia [Cf. Hom. 30 inter 1] that these three are species of Penance.
I answer that, This is a division of penance as a virtue. Now it must be observed that every virtue acts in accordance with the time being, as also in keeping with other due circumstances, wherefore the virtue of penance has its act at this time, according to the requirements of the New Law.
Now it belongs to penance to detest one's past sins, and to purpose, at the same time, to change one's life for the better, which is the end, so to speak, of penance. And since moral matters take their species from the end, as stated in I-II, 1, 3; I-II, 18, 4,6, it is reasonable to distinguish various species of penance, according to the various changes intended by the penitent.
Accordingly there is a threefold change intended by the penitent. The first is by regeneration unto a new life, and this belongs to that penance which precedes Baptism. The second is by reforming one's past life after it has been already destroyed, and this belongs to penance for mortal sins committed after Baptism. The third is by changing to a more perfect operation of life, and this belongs to penance for venial sins, which are remitted through a fervent act of charity, as stated above (87, 2,3).
Reply to Objection 1. The penance which precedes Baptism is not a sacrament, but an act of virtue disposing one to that sacrament.
Reply to Objection 2. The penance which washes away mortal sins, washes away venial sins also, but the converse does not hold. Wherefore these two species of penance are related to one another as perfect and imperfect.
Reply to Objection 3. Before Baptism there are no venial sins without mortal sins. And since a venial sin cannot be remitted without mortal sin, as stated above (Question 87, Article 4), before Baptism, penance for mortal sins is not distinct from penance for venial sins.
Source: Summa Theologica (New Advent)