Thomas Aquinas, on Ps 18:12

“For thy servant keepeth them, and in keeping them there is a great reward.”

“"Offenses." Here he prays concerning the offenses of those who transgress the law. And first he sets forth the hiddenness of sins. Second, he asks for their remission, at "From my hidden ones." Third, he sets forth the reason for the petition, at "If they shall not have dominion over me," and so on. He says therefore: we frequently offend against the law, but "Who can understand sins?" As if to say, no one. And this for three reasons. First, because sin blinds the eyes of the sinner, hence small sins are not easily discerned: Wis. 2: "Their own malice has blinded them." Second, because sins are many: Ps. 39: "Evils without number have surrounded me; my iniquities have overtaken me, and I was not able to see." Likewise, because of their subtlety: Ps. 39: "They are multiplied above the hairs of my head," from their subtlety. But he does not say "sins" but "offenses." There is a twofold kind of sin: namely, of transgression -- and this is more easily known, because a man knows he has done a wicked act. And also of omission -- and this is difficult to know, because these do not bind at all times, but according to place and time. And distinguish: when it is the proper place and time, it is difficult to affirm -- and when alms should be given, how and where, and so on. Then he sets forth the remission of sins. Now there is a twofold kind of sin. One takes its beginning from ourselves, and this is principally original sin, and those which proceed from the corruption of the fomes, such as sins of the flesh; and these stain the soul, because they join it to earthly things. And therefore he says, "From my hidden ones cleanse me," that is, those which proceed from a hidden root, or which are done in secret, or which arise from a secret will: Eph. 5: "The things that are done by them in secret, it is shameful even to speak of."”

Imported from an open dataset — not yet checked against the printed edition.