3 “For You a hymn is meet, O God, in Sion”. That fatherland is Sion: Jerusalem is the very same as Sion; and of this name the interpretation ye ought to know. As Jerusalem is interpreted vision of peace, so Sion Beholding, that is, vision and contemplation. Some great inexplicable sight to us is promised: and this is God Himself that has built the city. Beauteous and graceful the city, how much more beauteous a Builder it has! “For You a hymn is meet, O God,” he says.
But where? “In Sion:” in Babylon it is not meet. For when a man begins to be renewed, already with heart in Jerusalem he sings, with the Apostle saying, “Our conversation is in the Heavens.” For “in the flesh though walking,” he says, “not after the flesh we war.” Already in longing we are there, already hope into that land, as it were an anchor, we have sent before, lest in this sea being tossed we suffer shipwreck. In like manner therefore as of a ship which is at anchor, we rightly say that already she has come to land, for still she rolls, but to land in a manner she has been brought safe in the teeth of winds and in the teeth of storms; so against the temptations of this sojourning, our hope being grounded in that city Jerusalem causes us not to be carried away upon rocks.
He therefore that according to this hope sings, in that city sings: let him therefore say, “For You a hymn is meet, O God, in Sion.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)