6 “When the wicked man departed from me, I knew him not”. I approved him not, I praised him not, he pleased me not. For we find the word “to know” occasionally used in Scripture, in the sense of “to be pleased.” For what is hidden from God, brethren? Does He know the just, and does He not know the unjust? What do you think of, that He does not know? I say not, what do you think; but what will you ever think, that He will not have seen beforehand? God knows all things, then; and yet in the end, that is in judgment after mercy, He says of some persons: “I will profess unto them, I never knew you; depart from Me, you workers of iniquity.” Was there any one He did not know? But what means, “I never knew you”? I acknowledge you not in My rule. For I know the rule of My righteousness: ye agree not with it, you have turned aside from it, you are crooked. Therefore He said here also: “When the wicked man departed from Me, I knew him not.”...Therefore, “when the wicked man departed from me,” that is, when the wicked man was unlike me, and was unwilling to imitate my paths, was unwilling in his wickedness to live as I had proposed myself for his imitation; “I knew him not.” What means, “I knew him not”? Not that I was ignorant of him, but that I did not approve him.
7. “Whoso privily slandered his neighbour, him I persecuted”. Behold the righteous persecutor, not of the man, but of the sin. “With the proud eye, and the insatiable heart, I did not feed.” What means, “I did not feed with”? I did not eat in common with such. Attend, beloved; since you are about to hear something wonderful. If he did not feed with this man, he did not eat with him; for to feed is to eat; how is it then that we find our Lord Himself eating with the proud? It was not only with those publicans and sinners, for they were humble: for they acknowledged their weakness, and asked for the physician. We find that He ate with the proud Pharisees themselves. A certain proud man had invited Him: it was the same who was displeased because a sinning woman, one of ill repute in the city, approached the feet of our Lord....That Pharisee was proud: the Lord ate with him; what is it therefore that he says? “With such an one I did not eat.” How does He enjoin unto us what He has not done Himself? He exhorts us to imitate Himself: we see that He ate with the proud; how does He forbid us to eat with the proud? We indeed, brethren, for the sake of reproof, abstain from communion with our brethren, and do not eat with them, that they may be reformed? We rather eat with strangers, with Pagans, than with those who hold with us, if we have seen that they live wickedly, that they may be ashamed, and amend; as the Apostle says, “And if any man obey not our word by this Epistle, note that man, and have no company with him, that he may be ashamed. Yet count him not as an enemy, but admonish him as a brother.” For the sake of healing others we usually do this; but nevertheless we often eat with many strangers and ungodly men.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)