Matthew 17:14 When they came to the crowd, a man approached Jesus and knelt before him. 15 “Lord, have mercy on my son,” he said. “He has seizures and is suffering greatly. He often falls into the fire or into the water. 16 I brought him to your disciples, but they could not heal him.” 
17 “O unbelieving and perverse generation,” Jesus replied, “how long shall I stay with you? How long shall I put up with you? Bring the boy here to me.” 18 Jesus rebuked the demon, and it came out of the boy, and he was healed from that moment. 
19 Then the disciples came to Jesus in private and asked, “Why couldn’t we drive it out?” 
20 He replied, “Because you have so little faith. I tell you the truth, if you have faith 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.”

We focus on this particular incident because the context tells us that Jesus had just given to the disciples power to cast out demons in Luke 9:1-2. (Compare the above verses from Matthew with Luke 9:37-45.) If they already had the authority over demons, then why couldn’t they cast out the demon? Jesus, to say the least, was displeased at their inability to heal the boy. In His reply, Jesus referred to their “little faith,” pointing out that if they had faith as a mustard seed, they could indeed cast out the demon or even “move a mountain.” What is this “mountain-moving faith?” 

In Mark 11:12-24 Jesus on His way to Jerusalem was hungry and looked for figs on a tree. He found none, and proceeded to curse the fig tree. The tree withered, and Peter, the adventurous disciple who had a penchant for doing what he saw his master doing, wondered out loud how Jesus did it.

Mark 11:22 “Have faith in God,” Jesus answered. 23 “I tell you the truth, if anyone says to this mountain, ‘Go, throw yourself into the sea,’ and does not doubt in his heart but believes that what he says will happen, it will be done for him.