1 The title of this Psalm is brief in number of words, but heavy in the weight of its mysteries. “To David himself against Goliath.” This battle was fought in the time of our fathers, and you, beloved, remember it with me from Holy Scripture....David put five stones in his scrip, he hurled but one. The five Books were chosen, but unity conquered. Then, having smitten and overthrown him, he took the enemy's sword, and with it cut off his head. This our David also did, He overthrew the devil with his own weapons: and when his great ones, whom he had in his power, by means of whom he slew other souls, believe, they turn their tongues against the devil, and so Goliath's head is cut off with his own sword.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)