(Mat 17:19 KJV) Then came the disciples to Jesus apart, and said, Why could not we cast him out?

(Mat 17:20 KJV) And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.

(Mat 17:21 KJV) Howbeit this kind goeth not out but by prayer and fasting.