Psalms 87:1

"Her foundations are upon the holy hills" (ver. 1). The Psalm had as yet said nothing of the city: it begins thus, and says, "Her foundations are upon the holy hills." Whose? There can be no doubt that foundations, especially among the hills, belong to some city. Thus filled with the Holy Spirit, and with many thoughts of love and longing for that city, as if after long internal meditation, that citizen bursts out, "Her foundations are upon the holy hills;" as if he had already said something concerning it. And how could he have said nothing on a subject, respecting which in his heart he had never been silent? For how could "her foundations" have been written, of which nothing had been said before? But, as I said, after long and silent travailing in contemplation of that city in his mind, crying to God, he bursts out into the ears of men thus: "Her foundations are upon the holy hills." And, supposing persons who heard to enquire of what city he spoke he adds, "the Lord loveth the gates of Sion." Behold, then, a city whose foundations are upon the holy hills, a city called Sion, whose gates the Lord loveth, as he adds, "above all the dwellings of Jacob." But what doth this mean, "her foundations on the holy hills"? What are the holy hills upon which this city is built? Another citizen tells us this more explicitly, the Apostle Paul: of this was the Prophet a citizen, of this the Apostle citizen: and they spoke to exhort the other citizens. But how are these, I mean the Prophets and Apostles, citizens? Perhaps in this sense; that they are themselves the hills, upon which are the foundations of this city, whose gates the Lord loveth Let then another citizen state this clearly, that I may not seem to guess. […]

Its foundations are in the holy mountains. He did not say beforehand whose, yet he said, Its foundations are in the holy mountains. The one who speaks is the prophet: the sons are of Korah. What Korah means, I have often stated: a skull, that is, the sons of the resurrection. Its foundations: whether of God, or certainly of the Church. And what are the foundations, if not the Father and the Son and the Holy Spirit? These foundations, then, are not in the valleys, but are in the mountains: and not on any mountains whatever, but on the holy mountains. Paul says: 'As a wise architect I have laid the foundation'—that is, faith in the Trinity. Then he also says in another place: 'For he was awaiting the city having foundations, whose builder and maker is God.' Its foundations are in the holy mountains. He said, In the holy mountains, because there are also other mountains that are not holy. Then it is also said in Isaiah: 'Upon the dark mountain lift up a sign.' And in another place it is said: 'Who are you, corrupt mountain?' It is also said in Jeremiah: 'Give glory to the Lord your God, before your feet stumble against the dark mountains.' You see, then, that there are also dark mountains: and it is said to us by Jeremiah that we should give glory to God before our feet stumble against the dark mountains. For if our feet strike against those mountains, we cannot give glory to the Lord. Its foundations are in the holy mountains. Whom can we call the foundations? The apostles. In them were the foundations: where the faith of the Church was first laid, there too the foundations were laid.

We ought to know that what we have interpreted of the Church can be understood also of our soul. For if Zion is interpreted as a watchtower, and our soul in no other way but always considers higher things: the true church, the true temple of Christ is nothing other than the human soul. The Church of Christ is nothing else but the souls of those who believe in Christ. 'You,' he says, 'are the temple of God, and the Spirit of God dwells in you.' Its foundations are in the holy mountains: that is, the foundations of God are nowhere but in the holy mountains, in the doctrines of the believing soul. The Lord loves the gates of Zion. Our Zion has very many gates: which, if we open them, the bridegroom enters in to us, and sleeps with us. Then he himself says: 'Behold, I stand before the door and knock: if anyone opens to me, I will enter and dine with him.' Glorious things are spoken of you, O city of God. A city is not one building, but consists of many buildings: so the Church and our Zion, unless it has many virtues, will not be the city of God. I will be mindful of Rahab and Babylon among those who know me. She who once was in the breadth of sins, afterward ascended into the remembrance of God. Will a man say to Zion, and a man is born in her? I speak to you a wondrous thing, but a true one. Our Zion, in which at times there are foreigners, and Tyre, and the Ethiopian: that watchtower, that harlot, that Rahab, that Babylon, she who spread apart her feet to everyone passing by, according to Ezekiel: that harlot, if she wills, suddenly becomes a virgin. She becomes a virgin, and conceives the Son of God, and gives birth. 'From the fear of you, O Lord, we conceived and were in labor; we brought forth the spirit of your salvation upon the earth.' See, then, that the harlot conceives from God and gives birth, and brings forth the Savior. […]

A psalm of a canticle, for the sons of Korah. The difference between a psalm and a canticle I have often stated. A psalm is spoken from the psaltery, but a canticle is uttered from the voice. A psalm has to do with the hands, that is, with works; a canticle with contemplation (theoria).

Holiness is the strength and stability of the church: it is this that will support it and keep it from sinking; not so much that it is built upon mountains as that it is built upon holy mountains--upon the promise of God, for the confirming of which he has sworn by his holiness, upon the sanctification of the Spirit, which will secure the happiness of all the saints.