Objection 1. It would seem that the virtues are not restored through penance. Because lost virtue cannot be restored by penance, unless penance be the cause of virtue. But, since penance is itself a virtue, it cannot be the cause of all the virtues, and all the more, since some virtues naturally precede penance, viz., faith, hope, and charity, as stated above (Question 85, Article 6). Therefore the virtues are not restored through penance.
Objection 2. Further, Penance consists in certain acts of the penitent. But the gratuitous virtues are not caused through any act of ours: for Augustine says (De Lib. Arb. ii, 18: In Ps. 118) that "God forms the virtues in us without us." Therefore it seems that the virtues are not restored through Penance.
Objection 3. Further, he that has virtue performs works of virtue with ease and pleasure: wherefore the Philosopher says (Ethic. i, 8) that "a man is not just if he does not rejoice in just deeds." Now many penitents find difficulty in performing deeds of virtue. Therefore the virtues are not restored through Penance.
On the contrary, We read (Luke 15:22) that the father commanded his penitent son to be clothed in "the first robe," which, according to Ambrose (Expos. in Luc. vii), is the "mantle of wisdom," from which all the virtues flow together, according to Wisdom 8:7: "She teacheth temperance, and prudence, and justice, and fortitude, which are such things as men can have nothing more profitable in life." Therefore all the virtues are restored through Penance.
I answer that, Sins are pardoned through Penance, as stated above (86, 1). But there can be no remission of sins except through the infusion of grace. Wherefore it follows that grace is infused into man through Penance. Now all the gratuitous virtues flow from grace, even as all the powers result from the essence of the soul; as stated in I-II, 110, 4, ad 1. Therefore all the virtues are restored through Penance.
Reply to Objection 1. Penance restores the virtues in the same way as it causes grace, as stated above (Question 86, Article 1). Now it is a cause of grace, in so far as it is a sacrament, because, in so far as it is a virtue, it is rather an effect of grace. Consequently it does not follow that penance, as a virtue, needs to be the cause of all the other virtues, but that the habit of penance together with the habits of the other virtues is caused through the sacrament of Penance.
Reply to Objection 2. In the sacrament of Penance human acts stand as matter, while the formal power of this sacrament is derived from the power of the keys. Consequently the power of the keys causes grace and virtue effectively indeed, but instrumentally; and the first act of the penitent, viz., contrition, stands as ultimate disposition to the reception of grace, while the subsequent acts of Penance proceed from the grace and virtues which are already there.
Reply to Objection 3. As stated above (Question 86, Article 5), sometimes after the first act of Penance, which is contrition, certain remnants of sin remain, viz. dispositions caused by previous acts, the result being that the penitent finds difficulty in doing deeds of virtue. Nevertheless, so far as the inclination itself of charity and of the other virtues is concerned, the penitent performs works of virtue with pleasure and ease. even as a virtuous man may accidentally find it hard to do an act of virtue, on account of sleepiness or some indisposition of the body.
Source: Summa Theologica (New Advent)