5 And that you may know what sort of mountains these be that stand around Jerusalem; where Scripture has mentioned good mountains, very rarely, and hardly, and perhaps never, does it fail instantly to mention the Lord also, or allude to Him at the same moment, that our hopes rest not in the mountains....Lest thou again should tarry in the mountains, he at once adds, “Even so the Lord stands round about His people:” that your hope might not lie in the mountains, but in Him who lights the mountains. For when He dwells in the mountains, that is, in the Saints, He Himself is round about His people; and He has Himself walled His people with a spiritual fortification, that it may not be moved for evermore.
But when Scripture speaks of evil mountains, it adds not the Lord unto them. Such mountains, we have already told you often, signify certain mighty, but evil, souls. For you are not to suppose, brethren, that heresies could be produced through any little souls. None save great men have been the authors of heresies; but in proportion as they were mighty, so were they evil, mountains. For they were not such mountains as would receive peace, that the hills might receive righteousness; but they received dissension from their father the devil.
There were therefore mountains: beware thou fly not to such mountains. For men will come, and say unto you, There is a great hero, there is a great man! How great was that Donatus! How great is Maximian! And a certain Photinus, what a great man he was! And Arius too, how illustrious he was! All these I have mentioned are mountains, but mountains that cause shipwreck....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)