When they came to the crowd a man approached, knelt down before him, and said, “Lord, have pity on my son, for he is a lunatic and suffers severely; often he falls into fire, and often into water. I brought him to your disciples, but they could not cure him.” Jesus said in reply, “O faithless and perverse generation, how long will I be with you? How long will I endure you? Bring him here to me.” Jesus rebuked him and the demon came out of him, and from that hour the boy was cured. Then the disciples approached Jesus in private and said, “Why could we not drive it out?” He said to them, “Because of your little faith. Amen, I say to you, if you have faith the size of a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”Matthew 17:14-20
The condition attributed to a demon in this passage is likely to be epilepsy, according to many commentators. See the footnote to Matthew 17:15 in the New American Bible (Revised Edition).
Related post: Growing in Faith