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....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)