Then came the disciples to Jesus apart, and said, Why could not we cast him out? And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Howbeit this kind goeth not out but by prayer and fasting.

Matthew 17:19-21

And when he was come into the house, his disciples asked him privately, Why could not we cast him out? And he said unto them, This kind can come forth by nothing, but by prayer and fasting.

Mark 9:28-29

Before I begin, I want to thank Brothers Matthew Arnold and Jonah Lowrance for their insights on this study and the blessed discussions we had in trying to interpret these passages. The heart of what I am about to present comes from discussion with these two brothers.

The reason that Jesus gives for the disciples’ inability to cast this particular demon was their unbelief. They had been able to cast out demons in the past, and it would seem they had no previous problems. But something was missing this time. 

First, we should notice that by the time Jesus came on the scene, the disciples were engaged with Scribes while this man and his son were alone. When the disciples had difficulty, they should have prayed and possibly fasted for help from God. At the least, they should have kept them close so that they could get them to Jesus when He came back. But there is no record of them trying anything different. This is where we can see their unbelief being made manifest. We know that faith works by love (Galatians 5:6). Seeing that the man had to bring his son to Jesus by himself we can understand that the disciples had at least lacked the compassion they should have had. Therefore, their faith was not being used. I think we can safely assume that the disciples had confidence that they could help the man’s son but when they failed, they abandoned their faith. Perhaps they had more faith in their faith than they had in God? Perhaps they forgot that it was not their faith that had healed others. It was the power of God.

Another detail to consider is how Jesus healed the boy compared to how He told the disciples to heal him. No doubt, this was a special case. Jesus says as much. Jesus simply speaks and the demon comes out of the boy. The disciples had done the same thing before and it had worked. However, this time it did not work. Jesus tells them that from their perspective the only way to have this type of demon cast out was by imploring God for help. After all, it was a deaf and dumb demon. It possibly could not hear natural voices but the spiritual voice of God can speak wherever He wants!

Although the details about what is exactly happening here are difficult to see, we can still see the contrast in the faith of father and that of the disciples. The father clearly had faith in Jesus, Whom he believed could even help with his doubts and unbelief. The disciples seemed to give up on bringing things to Jesus when it got difficult and detractors came. 

We should learn from this that trusting in God is the right thing to do but that does not mean that life suddenly becomes easier. In fact, when we do trust God and move toward Him, the devil doubles down on his attacks. Therefore, we should pray more, fast more, and trust God more!