He will not suffer thy foot to stumble. Here the Prophet, in order to recall the faithful to the right path, and to defeat the influence of all the allurements which are wont to distract their minds, affirms that whatever advantages worldly men are accustomed to desire or hope for from the world, true believers will find abundantly and at hand in God alone. He not only attributes power to God, but also teaches that He is so affectioned towards us, that he will preserve us in all respects in perfect safety. As often as the power of God is extolled, there are many who immediately reply, It is very true that he can do such and such things if he is so inclined, but we do not certainly know what is his intention. In this passage, therefore, God is exhibited to the faithful as their guardian, that they may rest with assured confidence on his providence. As the Epicureans, in imagining that God has no care whatever about the world, extinguish all piety, so those who think that the world is governed by God only in a general and confused manner, and believe not that he cherishes with special care each of his believing people, leave men's minds in suspense, and are themselves kept in a state of constant fluctuation and anxiety. In short, never will the hearts of men be led in good earnest to call upon God, until a persuasion of the truth of this guardianship is deeply fixed in their minds. The Psalmist declares that the purpose for which God is our keeper, is, that he may hold us up. The Hebrew word, mvt, mot, which is here used, signifies both a sliding or falling, and a trembling or staggering. Now, although it often happens that the faithful stagger, yea, are even ready to fall altogether, yet as God sustains them by his power, they are said to stand upright. And as amidst the many dangers which every moment threaten us, it is difficult for us to get rid of all anxiety and fear, the Prophet at the same time testifies, that God keeps watch unceasingly over our safety.

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 Psalm 38:16. This, with the remainder of the psalm, seems to be of the nature of an answer to the anxious question in Psalm 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 Psalm 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.

He will not suffer thy foot to be moved - The foundation, God's infinite power and goodness, on which thou standest, cannot be moved; and whilst thou standest on this basis, thy foot cannot be moved.

He will not suffer thy foot to be moved: he that keepeth thee will not (b) slumber.
(b) He shows that God's providence not only watches over his Church in general, but also over every member of it.

He wilt not suffer thy foot to be moved,.... 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" slipped, and yet shall not fall finally and totally, or so as to perish; see Psalm 62:2;
he that keepeth thee will not slumber; neither angels nor men are the keepers of the saints, but the Lord himself; he is the keeper of every individual saint, of every regenerate person, of everyone of his sheep, of every member of his church; he keeps them by his power, he preserves them by his grace, he holds them with his right hand; guides them by his counsel, keeps their feet from falling, and brings them safe to glory: and a watchful keeper he is, he does not so much as slumber; he keeps them night and day, lest any harm them, Isaiah 27:3. Gussetius reads the whole as a prayer, "let him not suffer thy foot", &c. "let not thy keeper slumber" (i); to which the answer follows.
(i) "ne permittat--ne dormitet", Junius & Tremellius, Piscator; so Ainsworth.

His sleepless vigilance is added.
to be moved--(Compare Psalm 38:16; Psalm 66:9).