faith

 This happened right after the Transfiguration.

Verse 20 is the real kicker in this passage...

Matthew 17

14 At the foot of the mountain, a large crowd was waiting for them. A man came and knelt before Jesus and said, 15 “Lord, have mercy on my son. He has seizures and suffers terribly. He often falls into the fire or into the water. 16 So I brought him to your disciples, but they couldn’t heal him.”

17 Jesus said, “You faithless and corrupt people! How long must I be with you? How long must I put up with you? Bring the boy here to me.” 18 Then Jesus rebuked the demon in the boy, and it left him. From that moment the boy was well.

19 Afterward the disciples asked Jesus privately, “Why couldn’t we cast out that demon?”

20 “You don’t have enough faith,” Jesus told them. “I tell you the truth, if you had faith even as small as a mustard seed, you could say to this mountain, ‘Move from here to there,’ and it would move. Nothing would be impossible.”