John Calvin Commentary Psalms 121:4

"Behold, he that keepeth Israel Will neither slumber nor sleep." — Psalms 121:4 (ASV)

Behold! he who keeps Israel will not slumber nor sleep. To recall each individual to the consideration of the common covenant, he represents the Divine providence as extending to the whole body of the Church. So that each of us for himself may be assured that God will be gracious to him, we ought always to begin with the general promise made to all God’s people.

This form of expression, he will not slumber nor sleep, would be improper in other languages, according to whose idiom it should rather be, He will not sleep, indeed, he will not slumber: but when the Hebrews invert this order, they argue from the greater to the less.

The meaning, then, is that as God never slumbers even in the smallest degree, we need not be afraid of any harm happening to us while He is asleep. The Prophet's intention is now obvious. To persuade true believers that God has a special care for each of them in particular, he brings forward the promise that God made to the whole people, and declares God to be the guardian of His Church, so that from this general principle, as from a fountain, each might draw streams for himself.

Accordingly, immediately after (Psalms 121:5), addressing himself to each in particular, he repeats, Jehovah is your keeper, so that no one would hesitate to apply to himself what belonged to the whole community of Israel. Besides, God is called a defense at the right hand, to teach us that we do not need to go far to seek Him, but that He is near, or rather stands at our side to defend us.