18 But of the enemies themselves what? “Slay them not, lest sometime they forget Your law.” He is making request for his enemies, he is fulfilling the commandment....Slay not them of whom the sins Thou slayest. But what is it to be slain? To forget the law of the Lord. It is real death, to go into the pit of sin; this indeed may be also understood of the Jews. Why of the Jews, “Slay not them, lest sometime they forget Your law”? Those very enemies of mine, that have slain me, do not Thou slay.
Let the nation of the Jews remain: certes conquered it has been by the Romans, certes effaced is the city of them, Jews are not admitted into their city, and yet Jews there are. For all those provinces by the Romans have been subjugated. Who now can distinguish the nations in the Roman empire the one from the other, inasmuch as all have become Romans and all are called Romans? The Jews nevertheless remain with a mark; nor in such sort conquered have they been, as that by the conquerors they have been swallowed up.
Not without reason is there that Cain, on whom, when he had slain his brother, God set a mark in order that no one should slay him. This is the mark which the Jews have: they hold fast by the remnant of their law, they are circumcised, they keep Sabbaths, they sacrifice the Passover; they eat unleavened bread. These are therefore Jews, they have not been slain, they are necessary to believing nations. Why so? In order that He may show to us among our enemies His mercy. “My God has shown to me in mine enemies.” He shows His mercy to the wild-olive grafted on branches that have been cut off because of pride. Behold where they lie, that were proud, behold where you have been grafted, that lied: and be not proud, lest you should deserve to be cut off.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)