Psalm 91 is not addressed to God. It is, rather, an instructive or “wisdom” psalm intended to direct the life of the reader. It is grounded in the promises of God Himself (quoted in verses 14 – 16).
Life is full of sickness, traps, and the oppression of forces intent on control. But he who makes God his refuge, need not fear these things. He has given His angel the command to protect and preserve.
Perhaps a word is in order about “testing.” Just because God says “you will tread upon the lion and the cobra” doesn’t mean that one should court disaster by picking up snakes or befriending the king of beasts. This is, in essence, what the devil was tempting Jesus to do when he cited this psalm to get Jesus to try and prove his sonship (Matthew 4:6). Jesus’ reply was: “Do not put the Lord your God to the test.”
Finally, what if God doesn’t protect us, if we become ensnared by the traps of the world? Does it mean God has rejected us? Perhaps. There are all kinds of reasons for misfortune, but the sufferer should ask himself if it is true, that he has lived making God his refuge, trusting in Him.
Might misfortune mean God has failed in His promise? It will certainly seem that way.
But this psalm, as a piece of wisdom, is not offering an iron clad guarantee, only that the secure life can be counted on by the one who loves the Lord. Is there security for the one who does not?
Simply, no. This then is the only way that makes sense.