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 Psalms 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.