Albert Barnes Commentary Psalms 125:1

"They that trust in Jehovah Are as mount Zion, which cannot be moved, but abideth for ever." — Psalms 125:1 (ASV)

Those who trust in the Lord - His people; His friends. It is, and has always been, a characteristic of the people of God that they trust or confide in Him.

Will be like mount Zion - The mountain which David fortified, and on which the city was at first built (2 Samuel 5:6–9). The name Zion also became the name by which the entire city was known.

Which cannot be removed, but abides forever - A mountain is an emblem of firmness and stability, and it is natural to speak of it as something that could not be removed.

However, something more than this is intended here, as there is some basis for comparison especially regarding Mount Zion. This must have been either the idea that Zion was particularly strong by position, or that it was under divine protection and therefore safe. Most probably, it refers to Zion as a place secure by nature and made even more so by human construction.