17 Here is interposed, “The song of the diapsalma”: as it were the hidden joy, as far as we can imagine, of the separation which is now made, not in place, but in the affections of the heart, between sinners and the righteous, as of the grain from the chaff, as yet on the floor. And then follows, “Let the sinners be turned into hell”: that is, let them be given into their own hands, when they are spared, and let them be ensnared in deadly delight. “All the nations that forget God.” Because “when they did not think good to retain God in their knowledge, God gave them over to a reprobate mind.”
18. “For there shall not be forgetfulness of the poor man to the end”; who now seems to be in forgetfulness, when sinners are thought to flourish in this world's happiness, and the righteous to be in travail: but “the patience,” says He, “of the poor shall not perish for ever.” Wherefore there is need of patience now to bear with the evil, who are already separated in will, till they be also separated at the last judgment.
19. “Arise, O Lord, let not man prevail”. The future judgment is prayed for: but before it come, “Let the heathen,” says he, “be judged in Your sight:” that is, in secret; which is called in God's sight, with the knowledge of a few holy and righteous ones. “Place a lawgiver over them, O Lord.”. He seems to me to point out Antichrist: of whom the Apostle says, “When the man of sin shall be revealed.” “Let the heathen know that they are men.” That they who will be set free by the Son of God, and belong to the Son of Man, and be sons of men, that is, new men, may serve man, that is, the old man the sinner, “for that they are men.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)