4 What is this Jerusalem? He briefly describes it. “The mountains stand around Jerusalem”. Is it anything great, that we are in a city surrounded by mountains? Is this the whole of our happiness, that we shall have a city which mountains surround? Do we not know what mountains are? Or what are mountains save swellings of the earth? Different then from these are those mountains that we love, lofty mountains, preachers of truth, whether Angels, or Apostles, or Prophets.
They stand around Jerusalem; they surround her, and, as it were, form a wall for her. Of these lovely and delightful mountains Scripture constantly speaks....They are the mountains of whom we sing: “I lifted up my eyes unto the mountains, from whence my help shall come:” because in this life we have help from the holy Scriptures. And through the mountains that receive peace, the little hills received righteousness: for what says he of the mountains themselves? He said not, they have peace from themselves, or they make peace, or generate peace; but, they receive peace.
The Lord is the source, whence they receive peace. So therefore lift up your eyes to the mountains for the sake of peace, that your help may come from the Lord, who has made heaven and earth. Again, the Holy Spirit mentioning these mountains says this: “Thou dost light them wonderfully from Your everlasting mountains.” He said not, the mountains light them: but, Thou lightest them from Your everlasting mountains: through those mountains whom You have willed to be everlasting, preaching the Gospel, Thou lighting them, not the mountains. Such then are the “mountains that stand around Jerusalem.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)