He who dwells in the shelter of the Most High will abide in the shadow of the Almighty. ²I will say to the LORD, "My refuge and my fortress, my God, in whom I trust."—Psalm 91:1-2
Augustine begins his commentary on this passage by noting its unique use in the New Testament—unique not in its meaning, but in who speaks it. Psalm 91:11 is the verse that Satan quotes to Jesus in the wilderness temptation (Matthew 4:1–11). Augustine sees this connection as a call for our vigilance. If Satan dared to tempt our Lord with these words, might he not also tempt us?
Augustine encourages us to persevere through temptation even when the tempter is less obvious, when we are “assailed by the hidden attack of the devil.”
For many men are brave, when they are enduring persecution from men, and see them openly rage against themselves: imagining they are then imitating the sufferings of Christ, in case men openly persecute them; but if assailed by the hidden attack of the devil, they believe they are not being crowned by Christ. Never fear when thou dost imitate Christ. For when the devil tempted our Lord, there was no man in the wilderness; he tempted Him secretly; but he was conquered, and conquered too when openly attacking Him. This do thou, if thou wishest to enter by the door, when the enemy secretly assails thee, when he asks for a man that he may do him some hurt by bodily troubles, by fever, by sickness, or any other bodily sufferings, like those of Job. He saw not the devil, yet he acknowledged the power of God. He knew that the devil had no power against him, unless from the Almighty Ruler of all things he received that power: the whole glory he gave to God, power to the devil he gave not (Expositions on the Book of Psalms, 446).
Know that in whatever affliction you are suffering—especially when you're alone in really understand your struggle—that God is in control and no temptation comes your way apart from His good, omnipotent hand.