2 And see ye the names of those two cities, Babylon and Jerusalem. Babylon is interpreted confusion, Jerusalem vision of peace. Observe now the city of confusion, in order that you may perceive the vision of peace; that you may endure that, sigh for this. Whereby can those two cities be distinguished? Can we anywise now separate them from each other? They are mingled, and from the very beginning of mankind mingled they run on unto the end of the world. Jerusalem received beginning through Abel, Babylon through Cain: for the buildings of the cities were afterwards erected. That Jerusalem in the land of the Jebusites was built: for at first it used to be called Jebus, from thence the nation of the Jebusites was expelled, when the people of God was delivered from Egypt, and led into the land of promise. But Babylon was built in the most interior regions of Persia, which for a long time raised its head above the rest of nations. These two cities then at particular times were built, so that there might be shown a figure of two cities begun of old, and to remain even unto the end in this world, but at the end to be severed. Whereby then can we now show them, that are mingled? At that time the Lord shall show, when some He shall set on the right hand, others on the left. Jerusalem on the right hand shall be, Babylon on the left....Two loves make up these two cities: love of God makes Jerusalem, love of the world makes Babylon. Therefore let each one question himself as to what he loves: and he shall find of which he is a citizen: and if he shall have found himself to be a citizen of Babylon, let him root out cupidity, implant charity: but if he shall have found himself a citizen of Jerusalem, let him endure captivity, hope for liberty....Now therefore let us hear of, brethren, hear of, and sing of, and long for, that city whereof we are citizens. And what are the joys which are sung of to us? In what manner in ourselves is formed again the love of our city, which by long sojourning we had forgotten? But our Father has sent from thence letters to us, God has supplied to us the Scriptures, by which letters there should be wrought in us a longing for return: because by loving our sojourning, to enemies we had turned our face, and our back to our fatherland. What then is here sung?
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.”...
4. “And to You shall there be paid a vow in Jerusalem.” Here we vow, and a good thing it is that there we should pay. But who are they that here do vow and pay not? They that persevere not even unto the end in that which they have vowed. Whence says another Psalm, “Vow ye, and pay ye unto the Lord your God:” and, “to You shall it be paid in Jerusalem.” For there shall we be whole, that is, entire in the resurrection of just men: there shall be paid our whole vow, not soul alone, but the very flesh also, no longer corruptible, because no longer in Babylon, but now a body heavenly and changed. What sort of change is promised? “For we all shall rise again,” says the Apostle, “but we shall not all be changed....Where is, O death, your sting?” For now while there begin in use the first-fruits of the mind, from whence is the longing for Jerusalem, many things of corruptible flesh do contend against us, which will not contend, when death shall have been swallowed up in victory. Peace shall conquer, and war shall be ended. But when peace shall conquer, that city shall conquer which is called the vision of peace. On the part of death therefore shall be no contention. Now with how great a death do we contend! For thence are carnal pleasures, which to us even unlawfully do suggest many things: to which we give no consent, but nevertheless in giving no consent we contend....
5. “Hearken,” he says, “to my prayer, unto You every flesh shall come”. And we have the Lord saying, that there was given to Him “power over every flesh.” That King therefore began even now to appear, when there was being said, “Unto You every flesh shall come.” “To You,” he says, “every flesh shall come.” Wherefore to Him shall “every” flesh come? Because flesh He has taken to Him. Whither shall there come every flesh? He took the first-fruits thereof out of the womb of the Virgin; and now that the first-fruits have been taken to Him, the rest shall follow, in order that the holocaust may be completed. Whence then “every flesh”? Every man. And whence every man? Have all been foretold, as going to believe in Christ? Have not many ungodly men been foretold, that shall be condemned also? Do not daily men not believing die in their own unbelief? After what manner therefore do we understand, “Unto You every flesh shall come”? By “every flesh” he has signified, “flesh of every kind:” out of every kind of flesh they shall come to You. What is, out of every kind of flesh? Have there come poor men, and have there not come rich men? Have there come humble men, and not come lofty men? Have there come unlearned men, and not come learned men? Have there come men, and not come women? Have there come masters, and not come servants? Have there come old men, and not come young men; or have there come young men, and not come youths; or have there come youths, and not come boys; or have there come boys, and have there not been brought infants? In a word, have there come Jews (for thence were the Apostles, thence many thousands of men at first betraying, afterwards believing), and have there not come Greeks; or have there come Greeks, and not come Romans; or have there come Romans, and not come Barbarians? And who could number all nations coming to Him, to whom has been said, “Unto You every flesh shall come”?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)