John Calvin Commentary Psalms 91:5

"Thou shalt not be afraid for the terror by night, Nor for the arrow that flieth by day;" — Psalms 91:5 (ASV)

Thou shalt not fear for the terror of the night. The Psalmist continues to insist upon the truth to which I have just alluded: that if we rely with implicit confidence upon the protection of God, we will be secure from every temptation and assault of Satan. It is important to remember that those whom God has taken under His care are in a state of absolute safety.

Even those who have reached the most advanced experience find nothing more difficult than to rely upon Divine deliverance; especially when, overtaken by some of the many forms in which danger and death await us in this world, doubts will insinuate themselves into our hearts, causing fear and anxiety.

Therefore, the Psalmist had reason to specify different evils, encouraging the Lord’s people to look for more than one mode of deliverance and to bear up under various and accumulated calamities. Mention is made of the fear of the night because people are naturally apprehensive in the dark, or because the night exposes us to dangers of different kinds, and our fears are likely at such a time to magnify any sound or disturbance.

The arrow, rather than another weapon, is mentioned as flying by day, apparently because it shoots to a greater distance and with such swiftness that we can escape it only with difficulty. The verse that follows states the same truth, though in different words: that there is no kind of calamity that the shield of the Almighty cannot ward off and repel.