Augustine of Hippo

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" (ver. 2). 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 …

On the Psalms, Psalm 48 ↗

Ambrose of Milan

Truly the Lord is called great. For consider, he has poured exaltation and joy over all the orb of this earth where previously the land produced the most dreadful growth of sin; he has made joyful the earth that once abounded in sorrow, tears and groans. Now our conscience is set free from sin and can rejoice. Before this it was unhappy, it was drowned in the whirlpool of its own sin; but now, through the goodness and kindness of Christ, it is absolved from all error.

Commentaries on the Twelve Davidic Psalms, On Psalm 48 ↗

Thomas Aquinas

"It is founded." Here he commends the city in three ways. First, from its extent or joyfulness. Second, from its arrangement. Third, from the wisdom of its citizens. He says, therefore, "It is founded with the exultation of the whole earth." As if to say: it has been founded on the mountain, that is, Christ. But does this foundation pertain to one land only? No, but it overflows into the joy of the whole earth, because all share in the joy of this foundation: Ps. 65: "Shout joyfully to God, all the earth; sing a psalm," etc. Is. …

Exposition on the Psalms of David ↗