Psalms 121:3

3He 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, מוט, 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.