Psalm 91 is the polar opposite of Psalm 90.  This is because, if taken literally Psalm 91 would eliminate any need to complain or lament the way Psalm 90 does.  In Psalm 91, the Lord’s protection makes those who love God immune from injury or death in war, from disease, from being harmed by wild animals, or even from being subject to the effects of gravity!  Clearly, the people who wrote and canonized a psalm like this were not naïve.  They experienced reverses in warfare, were subject to the ravages of disease, did not blithely pretend that there was no such thing as gravity, and did not pretend that wild animals could be dangerous.  This is why the Psalm has to be carefully appropriated.  The Devil made no headway when he quoted this psalm when tempting Jesus.  Psalm 91 is not about whistling by the graveyard; instead, it is about hope.