1 The river Jordan, when they were entering across it into the land of promise, when touched by the feet of the priests who bore the Ark, stood still from above with bridled stream, while it flowed down from below, where it ran on into the sea, until the whole people passed over, the priests standing on the dry ground. We know these things, but yet we should not imagine in this Psalm, to which we have now answered by chanting Allelujah, that it is the purpose of the Holy Spirit, that while we call to mind those deeds of the past, we should not consider things like them yet to take place. For “these things,” as the Apostle says, “happened unto them for ensamples.”
2. “When Israel came out of Egypt, and the house of Jacob from among the strange people”, “Judah was His sanctuary, and Israel His dominion”; “the sea saw that and fled, Jordan was driven back”. Think not that past deeds are related unto us, but rather that the future is predicted; since, while those miracles also were going on in that people, things present indeed were happening, but not without an intimation of things future....Some things he has related differently to what we have learned and read there: that he might not truly be thought to be repeating past acts rather than to be prophesying future things. For in the first place, we read not that the Jordan was driven back, but that it stood still on the side nearest the source of its streams, while the people were passing through; next, we read not of the mountains and hills skipping: all which he has added, and repeated. For after saying, “The sea saw that, and fled; Jordan was driven back:” he added, “The mountains skipped like rams, and the little hills like young sheep”: and then asks, “What ails you, O thou sea, that you fled, and thou, Jordan, that you were driven back?”. “You mountains, that you skipped like rams; and you little hills, like young sheep?”.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)