If I must now explain how the just man “slays his enemies,” and prevails everywhere, it is to be observed that, when he says, “Every morning will I destroy the wicked of the land, that I may cut off all workers of iniquity from the city of Jehovah,” by “the land” he means the flesh whose lusts are at enmity with God; and by “the city of Jehovah” he designates his own soul, in which was the temple of God, containing the true idea and conception of God, which makes it to be admired by all who look upon it.
As soon, then, as the rays of the Sun of righteousness shine into his soul, feeling strengthened and invigorated by their influence, he sets himself to destroy all the lusts of the flesh, which are called “the wicked of the land,” and drives out of that city of the Lord which is in his soul all thoughts which work iniquity, and all suggestions which are opposed to the truth. And in this way also the just give up to destruction all their enemies, which are their vices, so that they do not spare even the children, that is, the early beginnings and promptings of evil.
In this sense also we understand the language of the 137th Psalm: “O daughter of Babylon, who art to be destroyed; happy shall he be that rewards you as you have served us: happy shall he be that takes and dashes your little ones against the stones.” For “the little ones” of Babylon (which signifies confusion) are those troublesome sinful thoughts which arise in the soul and he who subdues them by striking, as it were, their heads against the firm and solid strength of reason and truth, is the man who “dashes the little ones against the stones;” and he is therefore truly blessed.
God may therefore have commanded men to destroy all their vices utterly, even at their birth, without having enjoined anything contrary to the teaching of Christ; and He may Himself have destroyed before the eyes of those who were “Jews inwardly” all the offspring of evil as His enemies. And, in like manner, those who disobey the law and word of God may well be compared to His enemies led astray by sin; and they may well be said to suffer the same fate as they deserve who have proved traitors to the truth of God.
Source: Against Celsus (New Advent)