What does Mark 9:18 mean?

Perhaps the most important thing to understand about this verse is this: the modern disease of epilepsy is not caused by demon possession. We don't know why Jesus and His followers encounter so much demonic attack. Perhaps Satan is trying to derail their ministries. But we do know that not every illness, brain-related or otherwise, is the result of demons. The boy's symptoms are similar to epilepsy, including the inability to speak, tremors and jerking, and rigid muscles, but Matthew 4:24 differentiates between seizures caused by demons and those from more natural causes.

Scholars debate on the meaning of the word "rigid." The Greek root word is xeraino and often means to become dry or withered. That may mean the boy becomes pale and physically exhausted after an attack. This would certainly explain why the crowd thinks he is dead after the exorcism (Mark 9:26).

The disciples have expelled demons before (Mark 6:13). In fact, they were so successful they earned the attention of a crowd of five thousand that followed them to a desolate place outside of Bethsaida (Mark 6:32–33). Now, however, stuck between a desperate father and cynical scribes, they find themselves unable to help. Jesus tells the disciples that this type of demon can only be exorcised through prayer (Mark 9:29). More directly, He seems to criticize their lack of faith (Mark 9:19).

We risk this same decrease in power and effectiveness when we stop relying on God. In the beginning stages, when we have more faith and enthusiasm than ability and knowledge, God has the freedom to do great things through us. Later, we tend to take credit for the successes. It's often then that God removes His power to remind us how reliant on Him we really are.