Psalms 125:2

As the mountains are round about Jerusalem - Jerusalem, according to Sandys, was situated on a rocky mountain every way to be ascended, except a little on the north, with steep ascents and deep valleys, naturally fortified. It is surrounded with other mountains, at no great distance, as if placed in the midst of an amphitheatre; for on the east is Mount Olivet, separated from the city by the valley of Jehoshaphat, which also encompasses a part of the north; on the south, the mountain of Offiner interposed with the valley of Gehinnom; and on the west it was formerly fenced with the valley of Gihon, and the mountains adjoining. The situation was such as to be easily rendered impregnable.

The Lord is round about his people - He is above, beneath, around them; and while they keep within it, their fortress is impregnable, and they can suffer no evil.

As the mountains are round about Jerusalem - Hebrew, "Jerusalem - the mountains are round about her." Jerusalem, except on the north, is encompassed with hills or mountains, so that although the city was built on hills - Zion, Moriah, Bezethah, Acra - it was itself surrounded by hills higher than any of these, and was, in a certain sense, in a valley. See the notes at Matthew 2:1. Compare the notes at Psalm 48:1-14.

So the Lord is round about his people ... - As Jerusalem is thus protected by the hills around, so the people of God are protected by Yahweh. He surrounds the church; he is exalted far above the church; he guards the approaches to the church; he can defend it from all its foes. Under his protection it is safe. Jerusalem, as surrounded by hills and mountains, has thus become an emblem of the church at all times; its security was an emblem of the security of all who trust in the Lord.