3 Further,...when he had said, “in the city of our God, in His holy mountain,” what added he? “Spreading abroad the joys of the whole earth, the mountains of Sion”. Sion is one mountain, why then “mountains”? Is it that to Sion belonged also those which came from the other side, so as to meet together on the Corner Stone, and become two walls, as it were two mountains, one of the circumcision, the other of the uncircumcision; one of the Jews, the other of the Gentiles: no longer adverse, although diverse, because from different sides, now in the corner not even diverse.
“For He is our peace, who has made both one.” The same Corner Stone “which the builders rejected, has become the Head Stone of the corner.” The mountain has joined in itself two mountains; one house there is, and two houses; two, because coming from different sides; one, because of the Corner Stone, wherein both are joined together. Hear also this, “the mountains of Sion: the sides of the North are the city of the great King.”...See the Gentiles; “the sides of the North:” the sides of the North are joined to the city of the great King.
The North is wont to be contrary to Sion: Sion forsooth is in the South, the North over against the South. Who is the North, but He who said, “I will sit in the sides of the North, I will be like the Most High”? The devil had held dominion over the ungodly, and possessed the nations serving images, adoring demons; and all whatsoever there was of human kind anywhere throughout the world, by cleaving to Him, had become North. But since He who binds the strong man, takes away his goods, and makes them His own goods; men delivered from infidelity and superstition of devils, believing in Christ, are fitted on to that city, have met in the corner that wall that comes from the circumcision, and that was made the city of the great King, which had been the sides of the North.
Therefore also in another Scripture is it said, “Out of the North come clouds of golden colour: great is the glory and honour of the Almighty.” For great is the glory of the physician, when from being despaired of the sick recovers. “Out of the North come clouds,” and not black clouds, not dark clouds, not lowering, but “of golden colour.” Whence but by grace illumined through Christ? See, “the sides of the North are the city of the great King.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)