20 “If I have beheld iniquity in my heart, may not the Lord hearken”. Consider now, brethren, how easily, how daily men blushing for fear of men do censure iniquities; He has done ill, He has done basely, a villain the fellow is: this perchance for man's sake he says. See whether you behold no iniquity in your heart, whether perchance that which you censure in another, you are meditating to do, and therefore against him dost exclaim, not because he has done it, but because he has been found out. Return to yourself, within be to yourself a judge. Behold in your hid chamber, in the very inmost recess of the heart, where thou and He that sees are alone, there let iniquity be displeasing to you, in order that you may be pleasing to God. Do not regard it, that is, do not love it, but rather despise it, that is, contemn it, and turn away from it. Whatever pleasing thing it has promised to allure you to sin; whatever grievous thing it has threatened, to drive you on to evil doing; all is nought, all passes away: it is worthy to be despised, in order that it may be trampled upon; not to be eyed lest it be accepted....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)