Objection 1. It would seem that confession does not deliver one from the death of sin. For confession follows contrition. But contrition sufficiently blots out guilt. Therefore confession does not deliver one from the death of sin.
Objection 2. Further, just as mortal sin is a fault, so is venial. Now confession renders venial that which was mortal before, as stated in the text (Sent. iv, D, 17). Therefore confession does not blot out guilt, but one guilt is changed into another.
On the contrary, Confession is part of the sacrament of Penance. But Penance deliver from guilt. Therefore confession does also.
I answer that, Penance, as a sacrament, is perfected chiefly in confession, because by the latter a man submits to the ministers of the Church, who are the dispensers of the sacraments: for contrition has the desire of confession united thereto, and satisfaction is enjoined according to the judgment of the priest who hears the confession. And since in the sacrament of Penance, as in Baptism, that grace is infused whereby sins are forgiven, therefore confession in virtue of the absolution granted remits guilt, even as Baptism does. Now Baptism delivers one from the death of sin, not only by being received actually, but also by being received in desire, as is evident with regard to those who approach the sacrament of Baptism after being already sanctified. And unless a man offers an obstacle, he receives, through the very fact of being baptized, grace whereby his sins are remitted, if they are not already remitted. The same is to be said of confession, to which absolution is added because it delivered the penitent from guilt through being previously in his desire. Afterwards at the time of actual confession and absolution he receives an increase of grace, and forgiveness of sins would also be granted to him, if his previous sorrow for sin was not sufficient for contrition, and if at the time he offered no obstacle to grace. Consequently just as it is said of Baptism that it delivers from death, so can it be said of confession.
Reply to Objection 1. Contrition has the desire of confession attached to it, and therefore it delivers penitents from death in the same way as the desire of Baptism delivers those who are going to be baptized.
Reply to Objection 2. In the text venial does not designate guilt, but punishment that is easily expiated. and so it does not follow that one guilt is changed into another but that it is wholly done away. For "venial" is taken in three senses [Cf. I-II, 88, 2]: first, for what is venial generically, e.g. an idle word: secondly, for what is venial in its cause, i.e. having within itself a motive of pardon, e.g. sins due to weakness: thirdly, for what is venial in the result, in which sense it is understood here, because the result of confession is that man's past guilt is pardoned.
Source: Summa Theologica (New Advent)