2 The Psalm, then, begins thus: “O God, be not silent as to my praise; for the mouth of the ungodly, yea, the mouth of the deceitful is opened upon me”. Whence it appears, both that the blame, which the ungodly and the deceitful is not silent of, is false, and that the praise, which God is not silent of, is true. “For God is true, but every man a liar;” for no man is true, except him in whom God speaks. But the highest praise is that of the only-begotten Son of God, in which He is proclaimed even that which He is, the only-begotten Son of God. But this did not appear, but, when His weakness appeared, lay hid, when the mouth of the ungodly and deceitful was opened upon Him; and for this reason his mouth was opened, because His virtue was concealed: and he says, “the mouth of the deceitful was opened,” because the hatred which was covered by deceit burst out into language.
3. “They have spoken against me with false tongues”: then chiefly when they praised him as a “good Master” with insidious adulation. Whence it is elsewhere said: “and they that praised me, are sworn together against me.” Next, because they burst into cries, “Crucify Him, crucify Him;” he has added, “They compassed me about also with words of hatred.” They who with a treacherous tongue spoke words seemingly of love, and not of hatred, “against me,” since they did this insidiously; afterwards “compassed me about with words” not of false and deceitful love, but of open “hatred, and fought against me without a cause.” For as the pious love Christ for nought, so do the wicked hate Him for nought; for as truth is earnestly sought by the best men on its own account, without any advantage, external to itself, in view, so is wickedness sought by the worst men. Whence among secular authors it is said of a very bad man, “he was wicked and cruel for no object.”
4. “In place,” says he, “of loving me, they detracted from me”. There are six different acts of this class, which may, when mentioned, very easily be borne in mind; (1) to return good for evil, (2) not to return evil for evil; (3) to return good for good, (4) to return evil for evil; (5) not to return good for good, (6) to return evil for good. The two first of these belong to the good, and the first of these two is the better; the two last belong to the wicked, and the latter of the two is the worse; the two middle to a sort of middle class of persons, but the first of these borders upon the good, the latter on the bad. We should remark these things in the holy Scriptures. Our Lord Himself returns good for evil, who “justifies the ungodly;” and who, when hanging upon the Cross, said, “Father, forgive them; for they know not what they do.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)