John Calvin Commentary Psalms 121:6

"The sun shall not smite thee by day, Nor the moon by night." — Psalms 121:6 (ASV)

The sun shall not smite thee by day. By these forms of expression, the Psalmist magnifies the advantages that result for us from our having God present with us; and, by the figure synecdoche, under one particular, he declares in general that the faithful will be safe from all adversities, defended as they are by Divine power.

The language is metaphorical, the cold of night and the heat of day denoting all kinds of inconveniences. The sense then is that although God’s people may be subject, in common with others, to the miseries of human life, yet his shadow is always at their side to shield them from being harmed by them. The Prophet does not, however, promise the faithful a condition of such felicity and comfort as implies an exemption from all trouble; he only, to assuage their sorrows, sets before them this consolation: that being under Divine providence, they will be secure from all deadly harm—a point he explains more clearly in the following verses, where he tells us that God will so keep his own people from all evils as to maintain their life in safety. The statement in the text before us is indeed general, but he afterwards specifies the chief parts of human life.