If we have the faith as small as a grain of mustard seed, we can move mountains  -  Matthew 17:20

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.

The disciples were stunned at their lack of power. When they asked Jesus for an explanation, He told them it was because of their unbelief. If they had only the smallest amount of faith, they would be able to do great feats. They needed to understand that true faith must be based upon some command or promise of God, because if it is based on a feeling or whim, it is not faith but presumption. If God directs us to do something, no matter how big or small, we can have absolute confidence, we can do it.