1 Of this Psalm, the title seems not to need operose discussion: for simple and easy it appears. For thus it stands: “For the end, for David himself a Psalm of a Song.” But in many Psalms already we have reminded you what is “at the end: for the end of the Law is Christ for righteousness to every man believing:” He is the end which makes perfect, not that which consumes or destroys. Nevertheless, if any one endeavours to inquire, what means, “a Psalm of a Song:” why not either “Psalm” or “Song,” but both; or what is the difference between Psalm of Song, and Song of Psalm, because even thus of some Psalms the titles are inscribed: he will find perchance something which we leave for men more acute and more at leisure than ourselves....
2. “Let God rise up, and let His enemies be scattered”. Already this has come to pass, Christ has risen up, “who is over all things, God blessed for ever,” and His enemies have been dispersed through all nations, to wit, the Jews; in that very place, where they practised their enmities, being overthrown in war, and thence through all places dispersed: and now they hate, but fear, and in that very fear they do that which follows, “And let them that hate Him flee from His face.” The flight indeed of the mind is fear. For in carnal flight, whither flee they from the face of Him who everywhere shows the efficacy of His presence? “Whither shall I depart,” says he, “from Your Spirit, and from Your face whither shall I flee?” With mind, therefore, not with body, they flee; to wit, by being afraid, not by being hidden; and not from that face which they see not, but from that which they are compelled to see. For the face of Him has His presence in His Church been called....
3. “As smoke fails, let them fail”. For they lifted up themselves from the fires of their hatred unto the vapouring of pride, and against Heaven setting their mouth, and shouting, “Crucify, Crucify,” Him taken captive they derided, Him hanging they mocked: and being soon conquered by that very Person against whom they swelled victorious, they vanished away. “As wax melts from the face of fire, so let sinners perish from the face of God.” Though perchance in this passage he has referred to those men, whose hard-heartedness in tears of penitence is dissolved: yet this also may be understood, that he threatens future judgment; because though in this world like smoke, in lifting up themselves, that is, in priding themselves, they have melted away, there will come to them at the last final damnation, so that from His face they will perish for everlasting, when in His own glory He shall have appeared, like fire, for the punishment of the ungodly, and the light of the righteous.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)