Thomas Aquinas

“ As stated above, confession does not produce its effect, unless contrition be presupposed; concerning which no man can know whether it be true contrition, even as neither can one know for certain if he has grace. Consequently a man cannot know for certain whether a forgotten sin has been forgiven him in a general confession, although he may think so on account of certain conjectural signs. ”

“ Therefore if a man omits to confess a sin, because he does not know it to be a sin, through ignorance of the Divine law, he is not excused from insincerity. on the other hand, he would be excused, if he did not know it to be a sin, through being unaware of some particular circumstance, for instance, if he had knowledge of another's wife, thinking her his own. Now forgetfulness of an act of sin comes under the head of ignorance of fact, wherefore it excuses from the sin of insincerity in confession, which is an obstacle to the fruit of absolution and confession. ”

“ 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. ”

“ Which temporal punishment is beyond the powers of the penitent dwelling in this world, but is so far diminished by the power of the keys, that it is within the ability of the penitent, and he is able, by making satisfaction, to cleanse himself in this life. Secondly, confession diminishes the punishment in virtue of the very nature of the act of the one who confesses, for this act has the punishment of shame attached to it, so that the oftener one confesses the same sins, the more is the punishment diminished. ”

“ But he who confesses all the sins that he calls to mind, is not insincere through forgetting some, because he suffers from ignorance of fact, which excuses from sin. Therefore he receives forgiveness, and then the sins which he has forgotten, are loosened, since it is wicked to hope for half a pardon. ”

“ This suffices for the Reply to the First Objection.
Reply to Objection 2: The will is not taken for the deed, if this is done by another, as in the case of Baptism: for the will to receive Baptism is not worth as much as the reception of Baptism. ”