The sacred writers all delight in using strong images and figures, by which to represent divine things: as in this instance, God's covenant engagements in Christ are compared to strong mountains, the mountains of Jerusalem, fixed and immoveable. His promises indeed are all of this kind: yea and amen, 2 Corinthians 1:20. So sometimes the Lord represents his faithfulness and presence as a wall of fire round about, Zechariah 2:5. Such is Jesus, to encircle his people, so that they can never be approached for their hurt. The Lord sheaved this to the prophet's servant, when the invisible host of heaven, which is always taking its stand about the Lord's people, were made visible to his eyes, at the request of the prophet, 2 Kings 6:15-17.
Related Commentaries of Psalms 125:1
Psalms 125:1-2
1 They that trust in the LORD shall be as mount Zion, which cannot be removed, but abideth for ever.
2 As the mountains are round about Jerusalem, so the LORD is round about his people from henceforth even for ever.