In the gospel according to Matthew, the disciples could not heal the epileptic boy because

In the gospel according to Matthew, the disciples could not heal the epileptic boy because they had little faith. When the boy's father brought him to the disciples, they tried to cast out the demon that caused the boy's seizures, but they were not successful. Jesus later rebuked the disciples for their lack of faith, telling them that even a little faith is enough to move mountains. After Jesus cast out the demon, the disciples asked him privately why they were not able to do it, and he explained to them that it was because of their little faith.