1....The title of it causes us no delay, for it is both brief, and to be understood not difficult, especially to those nursed in the Church of God. For so it is, “To David himself.” The Psalm then is to David himself: now David is interpreted, Strong in hand, or Desirable. The Psalm then is to the Strong in hand, and Desirable, to Him who for us has overcome death, who unto us has promised life: for in this is He Strong in hand, that He has overcome death for us; in this is He Desirable, that He has promised unto us life eternal. For what stronger than that Hand which touched the bier, and he that was dead rose up? What stronger than that Hand which overcame the world, not armed with steel, but pierced with wood? Or what more desirable than He, whom not having seen, the Martyrs wished even to die, that they might be worthy to come unto Him? Therefore is the Psalm unto Him: to Him let our heart, to Him our tongue sing worthily: if yet Himself shall deign to give somewhat to sing....
2. “Judge Thou, O Lord” (says he), “them that hurt me, and fight Thou against them that fight against me”. “If God be for us, who can be against us?” And whereby does God this for us? “Take hold” (says he) “of arms and shield, and rise up to my help”. A great spectacle is it, to see God armed for you. And what is His Shield, what are His Arms? “Lord,” in another place says the man who here also speaks, “as with the shield of Your good-will have You compassed us.” But His Arms, wherewith He may not only us defend, but also strike His enemies, if we have well profited, shall we ourselves be. For as we from Him have this, that we be armed, so is He armed from us. But He is armed from those whom He has made, we are armed with those things which we have received from Him who made us. These our arms the Apostle in a certain place calls, “The shield of Faith, the helmet of Salvation, and the sword of the Spirit, which is the Word of God.” He has armed us with such arms as you have heard, arms admirable, and unconquered, insuperable and shining; spiritual truly and invisible, because we have to fight also against invisible enemies. If you see your enemy, let your arms be seen. We are armed with faith in those things which we see not, and we overthrow enemies whom we see not....
3. “Pour forth the weapon, and stop the way against them that persecute me”. Who are they that persecute you? Haply your neighbour, or he whom you have offended, or to whom you have done wrong, or who would take away what is yours, or against whom you preach the truth, or whose sin you rebuke, or whom living ill by your well living you offend. There are indeed even these enemies to us, and they persecute us: but other enemies we are taught to know, those against whom we fight invisibly, of whom the Apostle warns us, saying, “We wrestle not against flesh and blood,” that is, against men; not against those whom you see, but against those whom you see not; “against principalities, against powers, against the rulers of the world, of this darkness.”...“The whole world lies in wickedness;” therefore the Apostle explained of what world they were rulers, he said, “of this darkness.” The rulers of this world, I say, are the rulers of this darkness....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)