Jesus heals a demon-possessed boy, again predicts His own death, arranges to pay the Temple tax, tells the disciples a child is as great as they, warns against mistreating children, tells them how to discipline other believers, then tells the parables of the lost sheep and the unforgiving debtor: Mark 9:14–29; Matthew 17:14–21; Luke 9:37–45; Mark 9:30–32; Matthew 17:22–23; Matthew 17:24–27; Mark 9:33–37; Luke 9:46–50; Mark 9:38–50; Matthew 18
What does it mean: to not have enough faith?
When the Disciples were unable to cast a demon out of a boy, Jesus’ harsh words to His disciples were this:
Mark 9:19You faithless people! How long must I be with you? How long must I put up with you? Bring the boy to me.
This accusation was directed towards the Disciples, not the boy’s father. It certainly wasn’t directed towards the boy himself. Indeed, one man healed by Jesus (to the annoyance of the Pharisees) didn’t even know who He was; another set free from demons was too unwell to even ask (see Luke 8:26-39).
So what was Jesus saying?
Firstly, it was about faith in Jesus, not the miracles (or the ability to do them). A short conversation with the boy’s father showed the father both believed (in Jesus) but didn’t believe (that the boy could be healed after the first failure):
Mark 9:24 The father instantly cried out, ‘I do believe, but help me overcome my unbelief!’
This is where translations can make a difference. Other traditions translate this as help the defect of my faith, or help the weakness of my faith. The faith was there, but it wasn’t strong.
Secondly, it wasn’t the man’s faith that prevented the Disciples from setting the boy free. It was their faith.
Mark 9:28-29 Afterward, when Jesus was alone in the house with his disciples, they asked him, ‘Why couldn’t we cast out that evil spirit?’ Jesus replied, ‘This kind can be cast out only by prayer [and fasting].’
Or, to put it more bluntly, the Disciples’ lack of faith:
Matthew 13:20 You don’t have enough faith. I tell you the truth, if you had faith even as small as a mustard seed, you could say to this mountain, ‘Move from here to there,’ and it would move. Nothing would be impossible.
There is no record of Jesus not healing a person because the person didn’t have the faith that He could: all they had to do was have the faith to ask. Some, who were too unwell to ask, didn’t even need that.
Jesus was unable to do miracles (healing, casting out demons, raising the dead) only when people refused to believe Him at all.