Objection 1. It would seem that the forgiveness of guilt is not an effect of penance as a virtue. For penance is said to be a virtue, in so far as it is a principle of a human action. But human action does nothing towards the remission of guilt, since this is an effect of operating grace. Therefore the forgiveness of guilt is not an effect of penance as a virtue.
Objection 2. Further, certain other virtues are more excellent than penance. But the forgiveness of sin is not said to be the effect of any other virtue. Neither, therefore, is it the effect of penance as a virtue.
Objection 3. Further, there is no forgiveness of sin except through the power of Christ's Passion, according to Hebrews 9:22: "Without shedding of blood there is no remission." Now Penance, as a sacrament, produces its effect through the power of Christ's Passion, even as the other sacraments do, as was shown above (62, 4,5). Therefore the forgiveness of sin is the effect of Penance, not as a virtue, but as a sacrament.
On the contrary, Properly speaking, the cause of a thing is that without which it cannot be, since every defect depends on its cause. Now forgiveness of sin can come from God without the sacrament of Penance, but not without the virtue of penance, as stated above (84, 5, ad 3; 85, 2); so that, even before the sacraments of the New Law were instituted, God pardoned the sins of the penitent. Therefore the forgiveness of sin is chiefly the effect of penance as a virtue.
I answer that, Penance is a virtue in so far as it is a principle of certain human acts. Now the human acts, which are performed by the sinner, are the material element in the sacrament of Penance. Moreover every sacrament produces its effect, in virtue not only of its form, but also of its matter. because both these together make the one sacrament, as stated above (60, 6, ad 2, 7). Hence in Baptism forgiveness of sin is effected, in virtue not only of the form (but also of the matter, viz. water, albeit chiefly in virtue of the form) [The words in brackets are omitted in the Leonine edition] from which the water receives its power--and, similarly, the forgiveness of sin is the effect of Penance, chiefly by the power of the keys, which is vested in the ministers, who furnish the formal part of the sacrament, as stated above (Question 84, Article 3), and secondarily by the instrumentality of those acts of the penitent which pertain to the virtue of penance, but only in so far as such acts are, in some way, subordinate to the keys of the Church. Accordingly it is evident that the forgiveness of sin is the effect of penance as a virtue, but still more of Penance as a sacrament.
Reply to Objection 1. The effect of operating grace is the justification of the ungodly (as stated in I-II, 113), wherein there is, as was there stated (1,2,3), not only infusion of grace and forgiveness of sin, but also a movement of the free-will towards God, which is an act of faith quickened by charity, and a movement of the free-will against sin, which is the act of penance. Yet these human acts are there as the effects of operating grace, and are produced at the same time as the forgiveness of sin. Consequently the forgiveness of sin does not take place without an act of the virtue of penance, although it is the effect of operating grace.
Reply to Objection 2. In the justification of the ungodly there is not only an act of penance, but also an act of faith, as stated above (ad 1: I-II, 113, 4). Wherefore the forgiveness of sin is accounted the effect not only of the virtue of penance, but also, and that chiefly, of faith and charity.
Reply to Objection 3. The act of the virtue of penance is subordinate to Christ's Passion both by faith, and by its relation to the keys of the Church; and so, in both ways, it causes the forgiveness of sin, by the power of Christ's Passion.
To the argument advanced in the contrary sense we reply that the act of the virtue of penance is necessary for the forgiveness of sin, through being an inseparable effect of grace, whereby chiefly is sin pardoned, and which produces its effect in all the sacraments. Consequently it only follows that grace is a higher cause of the forgiveness of sin than the sacrament of Penance. Moreover, it must be observed that, under the Old Law and the law of nature, there was a sacrament of Penance after a fashion, as stated above (84, 7, ad 2).
Source: Summa Theologica (New Advent)