9 Now, brethren, you see the saints armed: observe the slaughter, observe their glorious battles. For if there be a commander, there must be soldiers; if soldiers, an enemy; if a warfare, a victory. What have these done who had in their hands swords sharpened on both sides? “To do vengeance on the nations.” See whether vengeance have not been done on the nations. Daily is it done: we do it ourselves by speaking. Observe how the nations of Babylon are slain. She is repaid twofold: for so is it written of her, “repay her double for what she has done.” How is she repaid double?
The saints wage war, they draw their “swords twice sharpened;” thence come defeats, slaughters, severances: how is she repaid double? When she had power to persecute the Christians, she slew the flesh indeed, but she crushed not God: now she is repaid double, for the Pagans are extinguished and the idols are broken....And lest you should think that men are really smitten with the sword, blood really shed, wounds made in the flesh, he goes on and explains, “upbraidings among the peoples.”
What is “upbraidings”? Reproof. Let the “sword twice sharpened” go forth from you, delay not. Say to your friend, if yet you have one left to whom to say it, “What kind of man are you, who hast abandoned Him by whom you were made, and worshippest what He made? Better is the Workman, than that which He works.” When he begins to blush, when he begins to feel compunction, you have made a wound with your sword, it has reached the heart, he is about to die, that he may live.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)