5 “He has set himself in every way that is not good.” What is, “he has set himself”? He has sinned perseveringly. Whence also of a certain pious and good man it is said, “He has not stood in the way of sinners.” As this “has not stood,” so that “has set himself.” “But wickedness has he not hated.” There is the end, there the fruit: if a man cannot but have wickedness, let him at least hate it. For when you hate it, it scarcely occurs to you to do any wickedness. For sin is in our mortal body, but what says the Apostle?
“Let not sin reign in your mortal body, that you should obey it in the lusts thereof.” When begins it not to be therein? When that shall be fulfilled in us which he says, “When this corruptible shall have put on incorruption, and this mortal shall have put on immortality.” Before this come to pass, there is a delighting in sin in the body, but greater is the delighting and the pleasure in the Word of Wisdom, in the Commandment of God. Overcome sin and the lust thereof. Sin and iniquity do thou hate, that you may join yourself to God, who hates it as well as thou.
Now being joined in mind unto the Law of God, in mind you serve the Law of God. And if in the flesh thou therefore servest the law of sin, because there are in you certain carnal delightings, then will there be none when you shall no longer fight. It is one thing not to fight, and to be in true and lasting peace; another to fight and overcome; another to fight and to be overcome; another not to fight at all, but to be carried away....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)