Matthew 17:19-20

19Then the disciples came to Jesus privately and asked, “Why could we not drive it out?”

20He answered, “Because of your little faith [your lack of trust and confidence in the power of God]; for I assure you and most solemnly say to you, if you have [living] faith the size of a mustard seed, you will say to this mountain, ‘Move from here to there,’ and [if it is God’s will] it will move; and nothing will be impossible for you.