So Jesus said to them, “Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you. However, this kind does not go out except by prayer and fasting.”
When Jesus, Peter, James and John returned from the mountain where the transfiguration occurred, they were greeted by a man whose son was severely troubled by a demon. The man had brought his son to the disciples, but “they could not cure him” (Matthew 17:16).
Jesus then commanded the demon to leave, and the boy was immediately cured (verse 18).
But the disciples were perplexed, since Jesus had previously given them power to heal the sick and cast out demons (Matthew 10:8). Why had they failed this time?
Jesus explained that they needed more faith. Faith in God is so powerful that, by analogy, even a tiny amount like a mustard seed could move mountains!
For more on the biblical teaching about faith, see the section on “Faith: Believing and Pleasing God.”