This is a wonderful explanation of Psalm 91 by John Piper from his book "A Godward Life", book two. I loved it so much I had to share it with you!
"No Evil Will Befall You."
Beware of Satan's Use of Psalms
The beloved Psalm 91 seems out of sync with experience and other Scripture. What are we to make of its promises?
A thousand may fall at your side and ten thousand at your right hand, but it shall not approach you. (verse 7)
No evil will befall you, nor will any plague come near your tent. (verse 10)
With a long life I will satisfy him. (verse 16)
Do people of faith never perish in battle? Do they never succumb to plague? Do they always live long lives?
There are three ways to resolve this problem. 1 ) We could say that the person who wrote the psalm and those who included it in the Psalter and the Bible had their heads in the sand and got it wrong. 2 ) Or we could say that those who fall in battle and get a disease and don't live long did not make the Lord their refuge and walk by faith. In other words, the promise is absolute and everyone who fails to experience it must be unbelieving. 3 ) Or we could say that what the writer means is that God does in fact rule the flight of arrows and the spread of disease and the length of life; and he can and does give safety and health and life to whom he pleases, so that it is always a free gift of God. But he does not mean for us to presume upon these promises as guarantees that God will not permit us to fall by an arrow, succumb to disease, or die at age thirty-eight. In other words, the promises have exceptions or qualifications.
When he says, "Ten thousand may fall at your right hand, but it shall not approach you" (verse 7), he means for you to understand this unspoken qualification: "It will not approach you without my permission and design. And my design for those who are ever in my care is always good, even if I permit the arrow to take their life." Thus Derek Kidner says, "This is a statement of exact, sweeping providence, not a charm against adversity...What it does assure us is that nothing can touch God's servant but by God's leave" (Psalms 73-150 [London: InterVarsity Press, 1975], 333).
There are several reasons in the context of the Psalter and the Bible why I think this was intended by the psalmist. The first is that even in the psalms themselves there are predictions that "many are the afflictions of the righteous" (34:19). And though the Lord delivers us "out of them all," yet we do go into them and may come out only in heaven (Psalm 63:3, NIV, "Your love is better than life.") Moreover in Psalm 44:22 those who have not been false to God's covenant nevertheless confess, "For your sake we are killed all day long; we are considered as sheep to be slaughtered," which is probably why Paul quotes this verse in Romans 8:36 about Christian martyrs, and then says, "But in all these things we are more than conquerors through Him who loved us" (8:37, NIV).
Perhaps most significant is the observation that Satan quotes Psalm 91:11-12 to Jesus in the wilderness (Matthew 4:6; Luke 4:10-11). How does Satan use this psalm? He uses it as though it had no qualifications. He takes Jesus to the pinnacle of the Temple and says to him, "If you are the Son of God, throw yourself down from here." And then he quotes Psalm 91: "For He will give His angels charge concerning you, to guard you in all your ways. They will bear you up in their hands, that you do not strike your foot against a stone" (verses 11-12). So Satan wants to exploit the very problem I am posing about this psalm. He is saying: "See! It is absolute. No qualifications. Use it. Prove the promise of God in your life! If it will apply to anyone, it will surely apply to you, the Son of God."
But Jesus rejects this use of the psalm and sets his face to prove that the psalm does have a qualification: He dies at a young age; he feels the blow of ripped flesh; and he is pierced by the nail and sword while ten thousand get off without a scratch. Jesus also teaches his disciples the paradoxical warning and promise: "They will put some of you to death... Yet not a hair of your head will perish (Luke 21:16-18). And Paul confirms this way of thinking, not only in Romans 8:28 and 35, but also in Philippians 4:19, where he says, "My God will supply all your needs," alongside the testimony, "I have learned the secret of...going hungry...and suffering need] through Him who strengthens me" (Philippians 4:12-13).
I urge you to follow Jesus' interpretation of Psalm 91, not Satan's. That is, in your Gethsemane of suffering, pray for deliverance according to God's sovereign power and mercy (twelve legions of angels could have rescued Jesus, Matthew 26:51). But then say, "Not my will but thine be done." And believe that whatever befalls will not, in the end, be evil for you, but good (Romans 8:28).