40 As to the Forgiveness of Sins, it ought to be enough simple to believe. For who would ask the cause or the reason when a Prince grants indulgence? When the liberality of an earthly sovereign is no fit subject for discussion, shall man's temerity discuss God's largess? For the Pagans are wont to ridicule us, saying that we deceive ourselves, fancying that crimes committed in deed can be purged by words. And they say, “Can he who has committed murder be no murderer, and he who has committed adultery be accounted no adulterer?
How then shall one guilty of crimes of this sort all of a sudden be made holy?” But to this, as I said, we answer better by faith than by reason. For he is King of all who has promised it: He is Lord of heaven and earth who assures us of it. Would you have me refuse to believe that He who made me a man of the dust of the earth can of a guilty person make me innocent? And that He who when I was blind made me see, or when I was deaf made me hear, or lame walk, can recover for me my lost innocence?
And to come to the witness of Nature— to kill a man is not always criminal, but to kill of malice, not by law, is criminal. It is not the deed then, in such matters, that condemns me, because sometimes it is rightly done, but the evil intention of the mind. If then my mind which had been rendered criminal, and in which the sin originated, is corrected, why should I seem to you incapable of being made innocent, who before was criminal? For if it is plain, as I have shown, that crime consists not in the deed but in the will, as an evil will, prompted by an evil demon, has made me obnoxious to sin and death, so the will prompted by the good God, being changed to good, has restored me to innocence and life.
It is the same also in all other crimes. In this way there is found to be no opposition between our faith and natural reason, while forgiveness of sins is imputed not to deeds, which when once done cannot be changed, but to the mind, which it is certain can be converted from bad to good.
Source: Commentary on the Apostles' Creed (New Advent)