Thou shalt not be afraid for the terror by night - That which usually causes alarm at night - a sudden attack; an unexpected incursion of enemies; sudden disease coming on by night; or the pestilence which seems to love night, and to “walk in darkness.” Any one of these things seems to be aggravated by night and darkness; and hence, we most dread them then. We cannot see their approach; we cannot measure their outlines; we know not the extent of the danger, or what may be the calamity.
Nor for the arrow that flieth by day - Whether shot from the bow of God - as pestilence and disease; or from the hand of man in battle. The idea is, that he that trusts in God will be calm. Compare the notes at Psalms 56:3.