John Calvin Commentary Psalms 121:3

"He will not suffer thy foot to be moved: He that keepeth thee will not slumber." — Psalms 121:3 (ASV)

He will not suffer your 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 usually 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 readily available in God alone.

He not only attributes power to God but also teaches that He is so affectionate towards us that he will preserve us in all respects in perfect safety. Whenever the power of God is praised, many 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 his intention is."

In this passage, therefore, God is presented to the faithful as their guardian, so that they may rest with assured confidence on his providence. Just as the Epicureans, by imagining that God has no care whatever about the world, extinguish all piety, so too, those who think that the world is governed by God only in a general and confused manner, and do not believe that he cherishes with special care each of his believing people, leave human minds in suspense and are themselves kept in a state of constant fluctuation and anxiety.

In short, human hearts will never be led to call upon God earnestly until the conviction 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, מוט, mot, which is used here, signifies both a sliding or falling, and a trembling or staggering.

Now, although it often happens that the faithful stagger, indeed, are even ready to fall altogether, yet because God sustains them by his power, they are said to stand upright.

And since, amid the many dangers that threaten us every moment, it is difficult for us to rid ourselves of all anxiety and fear, the Prophet at the same time testifies that God unceasingly watches over our safety.