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)