2 This Psalm is a “Song of degrees;” as we have often said to you, for these degrees are not of descent, but of ascent. He therefore longs to ascend. And whither does he wish to ascend, save into heaven? What means, into heaven? Does he wish to ascend that he may be with the sun, moon, and stars? Far be it! But there is in heaven the eternal Jerusalem, where are our fellow citizens, the Angels: we are wanderers on earth from these our fellow citizens. We sigh in our pilgrimage; we shall rejoice in the city. But we find companions in this pilgrimage, who have already seen this city herself; who summon us to run towards her. At these he also rejoices, who says, “I rejoiced in them who said unto me, We will go into the house of the Lord”...
3. “Our feet were standing in the courts of Jerusalem”...Consider what you will be there; and although you are as yet on the road, place this before your eyes, as if you were already standing, as if you were already rejoicing without ceasing among the Angels; as if that which is written were realized in you: “Blessed are they that dwell in Your house; they will be always praising You.” “Our feet stood in the courts of Jerusalem.” What Jerusalem? This earthly Jerusalem also is wont to be called by the name: though this Jerusalem is but the shadow of that. And what great thing is it to stand in this Jerusalem, since this Jerusalem has not been able to stand, but has been turned into a ruin? Does then the Holy Spirit pronounce this, out of the kindled heart of the loving Psalmist, as a great thing? Is not it that Jerusalem, unto whom the Lord said, “O Jerusalem, Jerusalem, you that killest the Prophets,” etc. What great thing then did he desire; to stand among those who slew the Prophets, and stoned them that were sent unto them? God forbid that he should think of that Jerusalem, who so loves, who so burns, who so longs to reach that Jerusalem, “our Mother,” of which the Apostle says, that She is “eternal in the Heavens.”
4. “Jerusalem that is being built as a city”. Brethren, when David was uttering these words, that city had been finished, it was not being built. It is some city he speaks of, therefore, which is now being built, unto which living stones run in faith, of whom Peter says, “You also, as lively stones, are built up a spiritual house;” that is, the holy temple of God. What means, you are built up as lively stones? You live, if you believe, but if you believe, you are made a temple of God; for the Apostle Paul says, “The temple of God is holy, which temple are you.” This city is therefore now in building; stones are cut down from the hills by the hands of those who preach truth, they are squared that they may enter into an everlasting structure. There are still many stones in the hands of the Builder: let them not fall from His hands, that they may be built perfect into the structure of the temple. This, then, is the “Jerusalem that is being built as a city:” Christ is its foundation. The Apostle Paul says, “Other foundation can no man lay than that is laid, which is Christ Jesus.” When a foundation is laid on earth, the walls are built above, and the weight of the walls tends towards the lowest parts, because the foundation is laid at the bottom. But if our foundation be in heaven, let us be built towards heaven. Bodies have built the edifice of this basilica, the ample size of which you see; and since bodies have built it, they placed the foundation lowest: but since we are spiritually built, our foundation is placed at the highest point. Let us therefore run there, where we may be built...But what Jerusalem do I speak of? Is it that, he asks, which you see standing, raised on the structure of its walls? No; but the “Jerusalem which is being built as a city.” Why not, a city, instead of, “as a city;” save because those walls, so built in Jerusalem, were a visible city, as it is by all called a city, literally; but this is being built “as a city,” for they who enter it are like living stones; for they are not literally stones? Just as they are called stones, and yet are not so: so the city styled “as a city,” is not a city; for he said, “is being built.” For by the word building, he meant to be understood the structure, and cohesion of bodies and walls. For a city is properly understood of the men that inhabit there. But in saying “is building,” he showed us that he meant a town. And since a spiritual building has some resemblance to a bodily building, therefore it “is building as a city.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)