when they had come to the multitude, there came to him a certain man, kneeling down to him and saying,
15 "Lord, have mercy on my son, for he is a lunatic, and suffers greatly; for he often falls into the fire, and into the water.
16 And I brought him to your disciples, and they could not cure him."
17 Then Jesus answered and said, "O faithless and perverse generation! How long will be I with you? How long will I bear with you? Bring him here to me."
18 And Jesus rebuked the devil, and he departed out of him; and the child was cured from that very hour.
19 Then the disciples came to Jesus privately and said, "Why could we not cast him out?"
20 And Jesus said to them, "Because of your unbelief; for truly I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, 'Move from here to there;' and it will move; and nothing will be impossible for you.
21 However, this kind only goes out by prayer and fasting."