There are, no doubt, passages in the New Testament which may seem at first sight to promise an invariable granting of our prayers. But that cannot be what they really mean. For in the very heart of the story we meet a glaring instance to the contrary. In Gethsemane the holiest of all petitioners prayed three times that a certain cup might pass from Him. It did not. After that the idea that prayer is recommended to us as a sort of infallible gimmick may be dismissed.
“Have faith in God,” Jesus answered. “Truly I tell you, if anyone says to this mountain, ‘Go, throw yourself into the sea,’ and does not doubt in their heart but believes that what they say will happen, it will be done for them. Therefore I tell you, whatever you ask for in prayer, believe that you have received it, and it will be yours.
C.S. Lewis, “The Efficacy of Prayer,” originally from The World’s Last Night and Other Essays, here from The Essential C.S. Lewis (Touchstone, 1996) 379.