Total believe or faith in Jesus so important

Matthew 17:14-20
[14]When they returned to the crowd, a man came to Jesus, knelt before him,
[15]and said, “Sir, have mercy on my son! He is an epileptic and has such terrible fits that he often falls in the fire or into water.
[16]I brought him to your disciples, but they could not heal him.”
[17]Jesus answered, “How unbelieving and wrong you people are! How long must I stay with you? How long do I have to put up with you? Bring the boy here to me!”
[18]Jesus gave a command to the demon, and it went out of the boy, and at that very moment he was healed.
[19]Then the disciples came to Jesus in private and asked him, “Why couldn’t we drive the demon out?”
[20]“It was because you haven’t enough faith,” answered Jesus. “I assure you that if you have faith as big as a mustard seed, you can say to this hill, ‘Go from here to there!’ and it will go. You could do anything!”