Luke 22:31-33 records: 'Simon, Simon, Satan has asked to sift you as wheat. But I have prayed for you, Simon, that your faith may not fail. And when you have turned back, strengthen your brothers.'

Since Peter's faith did 'fail', some argue, prayer must be worthless. However, God often permits trials for our spiritual growth, and one can hardly argue that things didn't turn out for the better. Peter's experience of failure undoubtedly was an experience that honed him into the powerhouse that he became.

Even so, this prayer did not fail—the word 'fail' (ekleipo), used only three times (twice in Luke and once in Hebrews), refers to a quite permanent condition. Peter's courage left him, but he never actually stopped believing in Jesus. Indeed, the passage assumes he will have a failure of some kind, and then turn around (he is told, 'when you have turned back,' meaning a conversion or turning around).