5 But perhaps it is only what is past that has been described to us: but nothing of the kind is to be hoped for by us for the future. Nay indeed, it is still to be hoped for. “Through You will we winnow away our enemies”. Our fathers then have declared to us a work that Thou did “in their days, and in the days of old,” that Your hand destroyed the Gentiles: that Thou “cast out the peoples; and planted them.” Such was the past; but what is to be hereafter? “Through You we shall winnow away our enemies.” A time will come, when all the enemies of Christians will be winnowed away like chaff, be blown like dust, and be cast off from the earth....Thus much of the future. “I will not trust in my bow,” even as our fathers did not in “their sword. Neither shall my sword help me”.
6. “For You have saved us from our enemies”. This too is spoken of the future under the figure of the past. But this is the reason that it is spoken of as if it were past, that it is as certain as if it were past. Give heed, wherefore many things are expressed by the Prophets as if they were past; whereas it is things future, not past facts that are the subject of prophecy. For the future Passion of our Lord Himself was foretold: and yet it says, “They pierced My hands and My feet. They told all My bones;” not, “They shall pierce,” and “shall tell.” “They looked and stared upon Me;” not “They shall look and stare upon Me.” “They parted My garments among them.” It does not say, “They shall part” them. All these things are expressed as if they were past, although they were yet to come: because to God things to come also are as certain as if they were past....It is for this reason, in consequence of their certainty, that those things which are yet future, are spoken of as if past. This it is then that we hope. For it is, “You have saved us from our enemies, and hast put them to shame that hated us.”
7. “In God will we boast all the day long”. Observe how he intermingles words expressive of a future time, that you may perceive that what was spoken of before as in past time was foretold of future times. “In God will we boast all day long; and in Your name will we confess for ever.” What is, “We shall boast”? What, “We shall confess”? That You have “saved us from our enemies;” that You are to give us an everlasting kingdom: that in us are to be fulfilled the words, “Blessed are they that dwell in Your house: they will be always praising You.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)