13 “Wilt not Thou, O God, that hast driven us back? And will not Thou, O God, march forth in our powers?”. Wilt not Thou lead us down, that hast driven us back? But wherefore “hast driven us back”? Because You have destroyed us. Wherefore hast destroyed us? Because angry You have been, and hast had pity on us. Thou therefore wilt lead down, that hast driven back; Thou, O God, that will not march forth in our powers, wilt lead down. What is, “will not march forth in our powers”?
The world is to rage, the world is to tread us down, there is to be a heap of witnesses, built of the spilled blood of martyrs, and the raging heathen are to say, “Where is the God of them?” Then “You will not march forth in our powers:” against them You will not show Yourself, You will not show Your power, such as You have shown in David, in Moses, in Joshua the son of Nun, when to their might the Gentiles yielded, and when the slaughter had been ended, and the great laying waste repaired, into the land which Thou promised Thou leddest in Your people.
This thing then You will not do, “You will not march forth in our powers,” but within You will work. What is, “will not march forth”? Wilt not show Yourself. For indeed when in chains the Martyrs were being led along, when they were being shut up in prison, when they were being led forth to be mocked, when to the beasts they were exposed, when they were being smitten with the sword, when with fire they were being burned, were they not despised as though forsaken, as though without helper?
In what manner was God working within? In what manner within was He comforting? In what manner to these men was He making sweet the hope of life everlasting? In what manner was He not forsaking the hearts of them, where the man was dwelling in silence, well if good, ill if evil? Was He then by any means forsaking, because He was not marching forth in the powers of them? By not marching forth in the powers of them, did He not the more lead down the Church even unto Idumæa, lead down the Church even unto the city of standing around? For if the Church chose to war and to use the sword, She would seem to be fighting for life present: but because she was despising life present, therefore there was made a heap of witness for the life that shall be.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)