1 The title of this Psalm is, “A song of praise, to the sons of Korah, on the second day of the week.” Concerning this what the Lord deigns to grant receive ye like sons of the firmament. For on the second day of the week, that is, the day after the first which we call the Lord's day, which also is called the second week-day, was made the firmament of Heaven....The second day of the week then we ought not to understand but of the Church of Christ: but the Church of Christ in the Saints, the Church of Christ in those who are written in Heaven, the Church of Christ in those who to this world's temptations yield not. For they are worthy of the name of “firmament.” The Church of Christ, then, in those who are strong, of whom says the Apostle, “We that are strong ought to bear the infirmities of the weak,” is called the firmament. Of this it is sung in this Psalm. Let us hear, acknowledge, associate, glory, reign. For Her called firmament, hear also in the Apostolic Epistles, “the pillar and firmament of the truth.”...
2. “Great is the Lord, and greatly to be praised”....That is, “in the city of our God, in His holy mountain.” This is the city set upon an hill, which cannot be hid: this is the candle which is not hidden under a bushel, to all known, to all proclaimed. Yet are not all men citizens thereof, but they in whom “great is the Lord, and greatly to be praised.” What then is that city: let us see whether perhaps, since it is said, “In the city of our God, in His holy mountain,” we ought not to enquire for this mountain where also we may be heard....What then is that mountain, brethren? One is it with great care to be enquired for, with great solicitude investigated, with labour also to be occupied and ascended. But if in any part of the earth it is, what shall we do? Shall we go abroad out of our own country, that to that mountain we may arrive? Nay, then we are abroad, when in it we are not. For that is our city, if we are members of the King, who is the head of the same city....For there was a certain corner-stone contemptible, whereat the Jews stumbled, cut out of a certain mountain without hands, that is, coming of the kingdom of the Jews without hands, because human operation went not with Mary of whom was born Christ. But if that stone, when the Jews stumbled thereat, had remained there, you had not had whither to ascend. But what was done? What says the prophecy of Daniel? What but that the stone grew, and became a great mountain? How great? So that it filled the whole face of the earth. By growing, then, and by filling the whole face of the earth, that mountain came to us. Why then seek we the mountain as though absent, and not as being present ascend to it; that in us the Lord may be “great, and greatly to be praised”?
Source: The Enarrations, or Expositions, on the Psalms (New Advent)