What does Luke 9:39 mean?

Three of Jesus' disciples—Peter, James, and John—have returned with Him from the Mount of Transfiguration. They find the other nine disciples unable to cast out a demon tormenting a little boy. This, despite Jesus giving them authority over all demons (Luke 9:1). The boy's father is explaining the situation to Jesus.

Matthew, Mark, and Luke relate different aspects of the boy's condition. Matthew says, "he has seizures and he suffers terribly" (Matthew 17:15). Mark says, "whenever it seizes him, it throws him down, and he foams and grinds his teeth and becomes rigid" (Mark 9:18). Luke combines the two, using the Greek word syntribō, translated as "shatter." This implies mauling, bruising, abuse, or damage. The three gospel writers don't contradict each other; the authors are just describing parts of the whole.

Throughout history, scholars have compared the boy's torment to epilepsy. Where Luke's "convulse" simply means to "shake to and fro," Matthew's word for "seizures" is the same used for "moonstruck:" the title ancients used for epilepsy.

The real correlation between demonization and epilepsy is unclear. People of Jesus' era believed epileptic symptoms were caused by demons. It is unknown if they associated demonic activity literally with the moon or with darkness in general. Neither Jesus nor Luke, a doctor, disavow the crowd's assumption that a demon is involved, and the symptoms disappear when Jesus casts out the demon (Luke 9:42).

There is no necessary contradiction, however, between epilepsy and demon possession. The passage isn't saying that epilepsy is inherently associated with demon possession. Nor is it saying that demons always cause illness, nor that demons cause every illness. In this case, the boy might have already had epilepsy, and the demon's affliction was to excessively trigger it. If that was the case, presumably Jesus also healed the epilepsy (Luke 9:42; Luke 5:17; 9:2, 11).