The traditional understanding of this statement is that a little true faith can see God do great things. The situation this father and his son were in was mountainous in size, and if the disciples would just exercise “mountain-moving faith” it could be dealt with. Even Paul in I Corinthians 13:2 speaks of faith removing mountains. The focus of this understanding tends to be on the enormity of the problem. Certainly there is truth in understanding that God can help us with enormous difficulties. But is that all that Jesus is saying here?
Then the disciples came to Jesus privately and said, “Why could we not drive it out?” And He said to them, “Because of the littleness of your faith; for truly I say to you, if you have faith the size of a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible to you. But this kind does not go out except by prayer and fasting.” (Matt. 17:19-21)
In Matthew 17 is recorded a situation most of us would do anything to avoid. A man brought to Jesus’ disciples his lunatic son. This son was so possessed by a demon that he convulsed and cast himself into fire and water in an attempt to burn or drown himself. Mark tells us in his gospel that this boy could not talk and that he was slammed to the ground periodically with his convulsions. When one of his fits seized him, he would foam at the mouth, grind his teeth, and stiffen like a board. Luke gives the further details that he screamed and the demon mauled him, scarcely leaving the poor boy alone. What a horrid situation.
And the disciples could not cast this demon out. It was a hopeless situation that they felt helpless to change. After Jesus rebukes the demon and heals this young man, the disciples ask Him the question above. Jesus responds with an answer that has become proverbial. He tells them that if they had faith as a mustard seed, they could move a mountain.