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.
2. “Blessed be the Lord my God, who teaches my hands for battle, my fingers for war”. These are our words, if we be the Body of Christ. It seems a repetition of sentiment; “our hands for battle,” and “our fingers for war,” are the same. Or is there some difference between “hands” and “fingers”? Certainly both hands and fingers work. Not then without reason do we take “fingers” as put for “hands.” But still in the “fingers” we recognise the division of operation, yet still a sort of unity. Behold that grace! The Apostle says, To one, this; to another, that; “there are diversities of operations; all these works one and the self-same Spirit;” there is the root of unity. With these “fingers” then the Body of Christ fights, going forth to “war,” going forth to “battle.”...By works of Mercy our enemy is conquered, and we could not have works of mercy unless we had charity, and charity we could have none unless we received it by the Holy Ghost; He then “teaches our hands for battle, and our fingers for war:” to Him rightfully do we say, “My Mercy,” from whom we have also that we are merciful: “for he shall have judgment without mercy, that has showed no mercy.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)