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)