Psalm 91 follows up on the words of Moses from Psalm 90 as we go back to the source: God. 

He is our refuge and with him he will protect us. We need not fear any tactic or timing or surprise of the enemy because God will turn the tables.

The irony and subtlety of these attacks is made all the more apparent when we consider that the devil quoted Psalm 91 to Jesus to tempt him. But yet the Lord Jesus trusts in his Father and allows. The Father to save him in his good timing, which is at the resurrection which Jesus gets satisfied. 

Indeed, the voice in this psalm is Jesus, who extends to us the honor of praying this psalm and to find favor and help from God as well.