7 “Nor for the arrow that flies by day, for the matter that walks in darkness, nor for the ruin and the devil that is in the noonday”. These two clauses above correspond to the two below; “You shall not fear” for “the terror by night, from the arrow that flies by day:” both because of “the terror by night,” from “the matter that walks in darkness:” and because of “the arrow that flies by day,” from “the ruin of the devil of the noon-day.” What ought to be feared by night, and what by day?
When any man sins in ignorance, he sins, as it were, by night: when he sins in full knowledge, by day. The two former sins then are the lighter: the second are much heavier; but this is obscure, and will repay your attention, if, by God's blessing, I can explain it so that you may understand it. He calls the light temptation, which the ignorant yield to, “terror by night:” the light temptation, which assails men who well know, “the arrow that flies by day.” What are light temptations?
Those which do not press upon us so urgently, as to overcome us, but may pass by quickly if declined. Suppose these, again, heavy ones. If the persecutor threatens, and frightens the ignorant grievously, I mean those whose faith is as yet unstable, and know not that they are Christians that they may hope for a life to come; as soon as they are alarmed with temporal ills, they imagine that Christ has forsaken them, and that they are Christians to no purpose; they are not aware that they are Christians for this reason, that they may conquer the present, and hope for the future: the matter that walks in darkness has found and seized them.
But some there are who know that they are called to a future hope; that what God has promised is not of this life, or this earth; that all these temptations must be endured, that we may receive what God has promised us for evermore; all this they know: when however the persecutor urges them more strenuously, and plies them with threats, penalties, tortures, at length they yield, and although they are well aware of their sin, yet they fall as it were by day.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)