The Hebrew word tsur β rock, fortress, refuge β appears over 70 times in the Psalms. David, who spent years fleeing from enemies, developed a theology of protection that was not theoretical. He had hidden in caves, crossed rivers at night, and watched armies pursue him. When he called God his rock and fortress, he was using fortress language because he had lived it. The protection of God is not a spiritual metaphor for people who've never needed actual safety.
Psalm 91 has been called the soldier's psalm, the plague psalm, the psalm for entering danger. It promises protection from pestilence, from the terror of night, from the arrow that flies by day. Charismatic tradition has always prayed this psalm over missionaries, over the sick, over those entering warfare. Not as a magic formula, but as a declaration of who God is to those who dwell in him.
The protection of God does not mean nothing hard will happen. The psalmist says "Though I walk through the valley of the shadow of death." Through. God doesn't always remove the valley β he walks through it with you. But Scripture also gives clear accounts of miraculous protection: the three men in the fiery furnace, Daniel in the lions' den, Paul shaking a viper into the fire and feeling nothing. The God who is with you in the danger is also the God who can eliminate it.
Commentary is from a charismatic Protestant perspective, drawing on KJV text and public-domain sources including Spurgeon, Andrew Murray, and Matthew Henry.