13 In this so great conflict, wherein man under Grace lives, and when, being aided, he fights well, rejoices in the Lord with trembling, there yet are not wanting even to valiant warriors, and mortifiers however unconquered of the works of the flesh, some wounds of sins, for the healing of which they may say daily, “Forgive us our debts:” against the same vices, and against the devil the prince and king of vices, striving with much greater watchfulness and keenness by the very prayer, that his deadly suggestions avail not anything, whereby he further urges the sinner to excuse rather than accuse his own sins; and thus those wounds not only be not healed, but also, although they were not deadly, yet may be pressed home to grievous and fatal harm.
And here therefore there is need of a more cautious Continence, whereby to restrain the proud appetite of man; whereby he is self-pleased, and unwilling to be found worthy of blame, and disdains, when he sins, to be convicted that he himself has sinned; not with healthful humility taking upon him to accuse himself, but rather with fatal arrogance seeking to find an excuse. In order to restrain this pride, he, whose words I have already set down above, and, as I could, commended, sought Continence from the Lord.
For, after that he had said, “Set, O Lord, a watch to my mouth, and a door of Continence around my lips. Make not my heart to fall aside unto evil words;” explaining more clearly whereof he spoke this, he says, “to make excuses in sins.” For what more evil than these words, whereby the evil man denies that he is evil, although convicted of an evil work, which he cannot deny. And since he cannot hide the deed, or say that it is well done, and still sees that it is clear that it was done by him, he seeks to refer to another what he has done, as though he could remove thence what he has deserved.
Being unwilling that himself be guilty, he rather adds to his guilt, and by excusing, not accusing, his own sins, he knows not that he is putting from him, not punishment, but pardon. For before human judges, forasmuch as they may be deceived, it seems to profit somewhat for the time, to cleanse as it were what has been done amiss by any deceit whatever; but before God, Who cannot be deceived, we are to use, not a deceitful defense, but a true confession of sins.
Source: On Continence (New Advent)