5 “You enlightening marvellously from the eternal mountains”. What are the eternal mountains? Those which He has Himself made eternal; which are the great mountains, the preachers of truth. You enlighten, but from the eternal mountains: the great mountains are first to receive Your light, and from Your light which the mountains receive, the earth also is clothed. But those great mountains the Apostles have received, the Apostles have received as it were the first streaks of the rising light....Wherefore also, in another place, a Psalm says what?
“I have lifted up my eyes unto the mountains, whence there shall come help to me.” What then, in the mountains is your hope, and from thence to you shall there come help? Have you stayed at the mountains? Take heed what you do. There is something above the mountains: above the mountains is He at whom the mountains tremble. “I have lifted up,” he says, “my eyes unto the mountains, whence there shall come help to me.” But what follows? “My help,” he says, “is from the Lord, who has made Heaven and earth.” Unto the mountains indeed I have lifted up eyes, because through the mountains to me the Scriptures were displayed: but I have my heart in Him that does enlighten all mountains....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)