What does Matthew 17:20 mean?The nine disciples left behind when Jesus took Peter, James, and John up the mountain (Matthew 17:1) have failed in a critical task. They could not cast a demon from a boy who was suffering greatly. Their public failure prompted both an argument with some Jewish scribes, as well as an exasperated response from Jesus about their "faithless and twisted" generation (Matthew 17:14–17).
Jesus healed the boy, but the disciples really want to understand what went wrong (Matthew 17:18–19). They have asked Him privately, away from the crowds, why they could not cast the demon out. Jesus responds to them directly, but relatively kindly in this verse. At the heart of their failure was their "little faith." They either did not trust the power previously given them by Jesus (Matthew 10:8) or they did not believe such power could be exercised through them. Perhaps they saw this specific case as too difficult. That they could perform miracles using Christ's power, at all, was an astounding reality. Jesus reveals here that the key to that power is faith in Him.
Part of the lesson here is the potency of genuine faith. In giving this reminder, Christ once again mentions the tiny mustard seed as a metaphor for living, active faith (Matthew 13:31–32). Even with that amount of faith, Jesus says, they will be able to tell a mountain to move and it will do so. With even a sliver of faith in His power and authority, nothing will be impossible for them.
This is an important and powerful lesson for the disciples. These men will eventually come to perform impossible-seeming miracles as they represent Jesus for the remainder of their lives on earth. The power to do so will never come from their own ability or goodness or status. It will always come through their special endowment, enabled by their faith in Jesus.