Objection 1. It would seem that hope of salvation should not be reckoned an effect of confession. For hope arises from all meritorious acts. Therefore, seemingly, it is not the proper effect of confession.
Objection 2. Further, we arrive at hope through tribulation, as appears from Romans 5:3-4. Now man suffers tribulation chiefly in satisfaction. Therefore, satisfaction rather than confession gives hope of salvation.
On the contrary," Confession makes a man more humble and more wary," as the Master states in the text (Sent. iv, D, 17). But the result of this is that man conceives a hope of salvation. Therefore it is the effect of confession to give hope of salvation.
I answer that, We can have no hope for the forgiveness of our sins except through Christ: and since by confession a man submits to the keys of the Church which derive their power from Christ's Passion, therefore do we say that confession gives hope of salvation.
Reply to Objection 1. It is not our actions, but the grace of our Redeemer, that is the principal cause of the hope of salvation: and since confession relies upon the grace of our Redeemer, it gives hope of salvation, not only as a meritorious act, but also as part of a sacrament.
Reply to Objection 2. Tribulation gives hope of salvation, by making us exercise our own virtue, and by paying off the debt of punishment: while confession does so also in the way mentioned above.
Source: Summa Theologica (New Advent)