What does Matthew 17:16 mean?

Jesus had taken Peter, James, and John with Him to the top of the mountain to witness His transfiguration (Matthew 17:1–13). During that time, He left the other nine core disciples (Matthew 10:1–4) behind at the foot of the mountain. In Mark's telling of this moment, Jesus returned to find a crowd gathered around His disciples as they engaged in an argument with some Jewish scribes (Mark 9:14–17).

The reason for the argument was that the disciples could not cast a demon out of a boy suffering from seizures. The combined details from Matthew, Mark, and Luke make it clear the boy's condition was not simply something like epilepsy, but something happening in especially dangerous moments (Matthew 17:15) thanks to a demonic spirit (Mark 9:17–18; Luke 9:39). The scribes were teachers of the Law and did not usually claim the power to cast out demons. Perhaps they were insisting that the disciples did not have the authority from God to cast out demons since they were unable to do so.

Jesus, though, had given the disciples the authority to do this. He had given them His authority and power to "heal the sick, raise the dead, cleanse lepers, cast out demons" to show that they represented Him and His kingdom on the earth (Matthew 10:8). In fact, these disciples may have cast out demons prior to this. So why did they fail to cast out this spirit? Jesus will explain it in the following verses.