Charles H. Spurgeon on Psalms 91:5

Thou shalt not be afraid. Not only do the pious stand safe, they are not even touched with fear. For the prophet does not say, Thou shalt not be seized; but, Thou shalt not be afraid. Certainly such a confidence of mind could not be attributed to natural powers, in so menacing and so overwhelming a destruction. For it is natural to mortals, it is implanted in them by God the author and maker of nature, to fear whatever is hurtful and deadly, especially what visibly smites and suddenly destroys. Therefore does he beautifully join together these two things: the first, in saying, Thou shalt not be afraid;the second, by adding, For the terror. He acknowledges that this plague is terrible to nature; and then by his trust in divine protection he promises himself this security, that he shall not fear the evil, which would otherwise make human nature quail. Wherefore, in my judgment, those persons are neither kind (humani) nor pious who are of opinion that so great a calamity is not to be dreaded by mortals. They neither observe the condition of our nature, nor honour the blessing of divine protection; both of which we see here done by the prophet. Musculus.