14And when He came to the disciples, He saw a great multitude around them, and scribes disputing with them. 15Immediately, when they saw Him, all the people were greatly amazed, and running to Him, greeted Him. 16And He asked the scribes, "What are you discussing with them?"
17Then one of the crowd answered and said, "Teacher, I brought You my son, who has a mute spirit. 18And wherever it seizes him, it throws him down; he foams at the mouth, gnashes his teeth, and becomes rigid. So I spoke to Your disciples, that they should cast it out, but they could not."
19He answered him and said, "O faithless generation, how long shall I be with you? How long shall I bear with you? Bring him to Me." 20Then they brought him to Him. And when he saw Him, immediately the spirit convulsed him, and he fell on the ground and wallowed, foaming at the mouth.
21So He asked his father, "How long has this been happening to him?"
And he said, "From childhood. 22And often he has thrown him both into the fire and into the water to destroy him. But if You can do anything, have compassion on us and help us."
23Jesus said to him, "If you can believe, all things are possible to him who believes."
24Immediately the father of the child cried out and said with tears, "Lord, I believe; help my unbelief!"
25When Jesus saw that the people came running together, He rebuked the unclean spirit, saying to it, "Deaf and dumb spirit, I command you, come out of him and enter him no more!" 26Then the spirit cried out, convulsed him greatly, and came out of him. And he became as one dead, so that many said, "He is dead." 27But Jesus took him by the hand and lifted him up, and he arose.
28And when He had come into the house, His disciples asked Him privately, "Why could we not cast it out?"
29So He said to them, "This kind can come out by nothing but prayer and fasting."
Jesus gave the disciples power over demons, to be able to cast them out. They had been successful doing this many times before. But now the scribes bring them a boy whose demon the disciples were unable to cast out.
Jesus makes it easy to see that the father of the boy has little faith. The father shows his lack of faith by requesting "But if you can do anything...". All things are possible to him to believes. It is after the father's turn to faith in "Lord, I believe; help my unbelief!" that Jesus heals the boy.
It was easier for the people to have faith when Jesus was in their presence, knowing Jesus could help them. Jesus expressed his worry about them once he was no longer walking among them. Would they be able to keep their faith? Jesus has stressed the importance of faith in much of his teachings. We all need faith.
Why were the disciples unable to cast out this demon? Jesus says it is because this kind can come out by nothing but prayer and fasting. I don't think Jesus meant that He would be unable to cast out this kind of demon without prayer and fasting. I think He is saying that the disciples would not be able to cast out this kind of demon on their own.
Just as the disciples were unable to cast out this demon, we cannot expect our church pastors and ministers to be able to fix all of our problems. Our faith is required in order for them to do their work to help us. It is through our prayer and fasting that we demonstrate our faith, or call for help for our faith as the father of this boy did. All things are possible for those who believe. We must have faith.