Objection 1. It would seem that confession is not an act of virtue. For every act of virtue belongs to the natural law, since "we are naturally capable of virtue," as the Philosopher says (Ethic. ii, 1). But confession does not belong to the natural law. Therefore it is not an act of virtue.
Objection 2. Further, an act of virtue is more befitting one who is innocent than one who has sinned. But the confession of a sin, which is the confession of which we are speaking now, cannot be befitting an innocent man. Therefore it is not an act of virtue.
Objection 3. Further, the grace which is in the sacraments differs somewhat from the grace which is in the virtues and gifts. But confession is part of a sacrament. Therefore it is not an act of virtue.
On the contrary, The precepts of the law are about acts of virtue. But confession comes under a precept. Therefore it is an act of virtue.
Further, we do not merit except by acts of virtue. But confession is meritorious, for "it opens the gate of heaven," as the Master says (Sent. iv, D, 17). Therefore it seems that it is an act of virtue.
I answer that, As stated above (I-II, 18, 6,7; II-II, 80; II-II, 85, 3; II-II, 109, 3), for an act to belong to a virtue it suffices that it be of such a nature as to imply some condition belonging to virtue. Now, although confession does not include everything that is required for virtue, yet its very name implies the manifestation of that which a man has on his conscience: for thus his lips and heart agree. For if a man professes with his lips what he does not hold in his heart, it is not a confession but a fiction. Now to express in words what one has in one's thoughts is a condition of virtue; and, consequently, confession is a good thing generically, and is an act of virtue: yet it can be done badly, if it be devoid of other due circumstances.
Reply to Objection 1. Natural reason, in a general way, inclines a man to make confession in the proper way, to confess as he ought, what he ought, and when he ought, and in this way confession belongs to the natural law. But it belongs to the Divine law to determine the circumstances, when, how, what, and to whom, with regard to the confession of which we are speaking now. Accordingly it is evident that the natural law inclines a man to confession, by means of the Divine law, which determines the circumstances, as is the case with all matters belonging to the positive law.
Reply to Objection 2. Although an innocent man may have the habit of the virtue whose object is a sin already committed, he has not the act, so long as he remains innocent. Wherefore the confession of sins, of which confession we are speaking now, is not befitting an innocent man, though it is an act of virtue.
Reply to Objection 3. Though the grace of the sacraments differs from the grace of the virtues, they are not contrary but disparate; hence there is nothing to prevent that which is an act of virtue, in so far as it proceeds from the free-will quickened by grace, from being a sacrament, or part of a sacrament, in so far as it is ordained as a remedy for sin.
Source: Summa Theologica (New Advent)