Psalms 121:3

“He wilt not suffer thy foot to be moved, etc.] This is either an address of the psalmist to his own soul; or to any other good man, his friend and acquaintance, assuring of stability, and of final perseverance in grace to glory. The Lord keeps the feet of his saints from falling: he will not suffer them to be moved out of the spiritual estate in which they stand; nor off of the Foundation and Rock of ages, on which their feet are set, and their goings established; nor out of the house of God, where they are as pillars; nor out of his ways, where he upholds their goings; moved in some sense they may be, yet not “greatly moved”; their feet may be “almost” gone, and their steps “well nigh”…”

“He will not suffer thy foot to be moved - He will enable you to stand firm. You are safe in his protection. Compare the notes at Psalms 38:16. This, with the remainder of the psalm, seems to be of the nature of an answer to the anxious question in Psalms 121:1 - an answer which the author of the psalm, in danger and trouble, makes to his own soul, imparting confidence to himself. He that keepeth thee will not slumber - He will be ever watchful and wakeful. Compare Isaiah 27:3. All creatures, as far as we know, sleep; God never sleeps. Compare Psalms 139:11-12. His eyes are upon us by day, and in the darkness of the night - the night literally; and also the night of calamity, woe, and sorrow.…”

“will not: Psa 91:12; Sa1 2:9; Pro 2:8, Pro 3:23, Pro 3:26; Pe1 1:5 Psalms 121:4”