Jesus rebuked the demon, and it came out of the boy, and he was healed from that moment.  Then the disciples came to Jesus in private and asked, “Why couldn’t we drive it out?”  He replied, “Because you have so little faith.  I tell you the truth, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there’ and it will move.  Nothing will be impossible for you. “

Matthew 17:18-20 New International Version