The disciples 'could not' drive out a demon. They tried and tried. How many demons did Jesus give them power and authority to drive out? All demons, yet they failed. Have you ever failed as a disciple and follower of Jesus? I certainly have. When I fail ministering to others, or fail personally, I've learned there's a spirit involved that I 'could not' handle. It's not that I didn't try. How about you? Ever fail as a Christian?