A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
psa 125:1God honors the confidence of His people, by protection and deliverance, and leaves hypocrites to the doom of the wicked. (Psa 125:1-5)
Mount Zion--as an emblem of permanence, and locality of Jerusalem as one of security, represent the firm and protected condition of God's people (compare Psa 46:5), supported not only by Providence, but by covenant promise. Even the mountains shall depart, and the hills be removed, but God's kindness shall not depart, nor His covenant of peace be removed (Isa 54:10).
They that trust--are "His people," (Psa 125:2).
psa 125:3Though God may leave them for a time under the "rod," or power (Psa 2:9), and oppression of the wicked for a time, as a chastisement, He will not suffer them to be tempted so as to fall into sin (Co1 10:13). The wicked shall only prove a correcting rod to them, not a destroying sword; even this rod shall not remain ("rest") on them, lest they be tempted to despair and apostasy (Psa 73:13-14). God may even try His people to the uttermost: when nothing is before our eyes but pure despair, then He delivers us and gives life in death, and makes us blessed in the curse (Co2 1:8-9) [LUTHER].
the lot--the possession, literally, "Canaan," spiritually, the heavenly inheritance of holiness and bliss which is appointed to the righteous. Sin's dominion shall not permanently come between the believer and his inheritance.
psa 125:4(Compare Psa 7:10; Psa 84:11).
psa 125:5Those who turn aside (under temptation) permanently show that they are hypocrites, and their lot or portion shall be with the wicked (Psa 28:3).
crooked ways--(Compare Deu 9:16; Mal 2:8-9).
their--is emphatic; the "crooked ways" proceed from their own hearts. The true Israel is here distinguished from the false. Scripture everywhere opposes the Jewish delusion that mere outward descent would save (Rom 2:28-29; Rom 9:6-7; Gal 6:16). The byways of sin from the way of life.