18 And then He tells what meanwhile, during this whole time when He already has risen, and remains still with the Father, He suffers by the intermixture of sinners in His Body, the Church, and by the separation of heretics. “If Thou, O God, shall slay the sinners (since You shall say in Your thought, Depart from Me, you men of blood), they shall receive in vanity their cities” (ver. 19-20). The words seem to be connected in this order; “If Thou, O God, shall slay the sinners, they shall receive in vanity their cities.”
Thus are sinners slain, because, “having their understandings darkened, they are alienated from the life of God.” For on account of elation they lose confession, and so they are slain, and in them is fulfilled what Scripture says, “Confession perishes from the dead, as from one that is not.” And so “they receive in vanity their cities,” that is, their vain peoples, who follow their vanity; when, puffed up by the name of righteousness, they persuade men to burst the bond of unity, and blindly and ignorantly follow them, as being more righteous....But now the Body of Christ, the Church, says, Why do the proud speak falsely against me, as though I were stained by other men's sins, and so, by separating themselves, “receive in vanity their cities”?
“Have not I hated those who hated You, Lord?”. Why do those who are worse themselves require of me to separate myself in body as well as spirit from the wicked, so as to root up the wheat, together with the tares, before the time of harvest, that before the time of winnowing I lose my power of enduring the chaff; that before all the different sorts of fishes are brought to the end of the world, as to the shore, to be separated, I tear the nets of peace and unity? Are the sacraments which I receive, those of evil men?
Do I; by consent, communicate in their life and deeds?...But where is, “Love your enemies”? Is it because He said “yours,” not “God's”? “Do good to them that hate you.” He says not, “who hate God.” So he follows the pattern, and says, “Have not I hated those who hated You; Lord?” He says not, “Who have hated me.” “And at Your enemies did I waste away.” “Yours,” he said, not “mine.” But those who hate us and are enemies unto us, only because we serve Him, what else do they but hate Him, and are His enemies.
Ought we then to love such enemies as these? Or do not they suffer persecution for God's sake, to whom it is said, “Pray for them that persecute you”? Observe then what follows. “With a perfect hatred did I hate them”. What is, “with a perfect hatred”? I hated in them their iniquities, I loved Your creation. This it is to hate with a perfect hatred, that neither on account of the vices thou hate the men, nor on account of the men love the vices. For see what he adds, “They became mine enemies.”
Not only as God's enemies, but as his own too does he now describe them. How then will he fulfil in them both his own saying, “Have not I hated those that hated You, Lord,” and the Lord's command, “Love your enemies”? How will he fulfil this, save with that “perfect hatred,” that he hate in them that they are wicked, and love that they are men? For in the time even of the Old Testament, when the carnal people was restrained by visible punishments, how did Moses, the servant of God, who by understanding belonged to the New Testament, how did he hate sinners when he prayed for them, or how did he not hate them when he slew them, save that he “hated them with a perfect hatred”? For with such perfection did he hate the iniquity which he punished, as to love the manhood for which he prayed.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)