Then the disciples came to Jesus privately, and said, "Why weren’t
we able to cast it out?"
He said to them, "Because of your unbelief. For most assuredly I tell you, if you have faith as a grain of mustard seed, you will tell this mountain, ‘Move from here to there,’ and it will move; and nothing will be impossible for you.
Mark 9:17-29 (web)
One of the multitude answered, "Teacher, I brought to you my son, who has a mute spirit;
and wherever it seizes him, it throws him down, and he foams at the mouth, and grinds his teeth, and wastes away. I asked your disciples to cast it out, and they weren’t able."
He answered him, "Unbelieving generation, how long shall I be with you? How long shall I bear with you? Bring him to me."
They brought him to him, and when he saw him, immediately the spirit convulsed him,
and he fell on the ground, wallowing and foaming at the mouth.
He asked his father, "How long has it been since this has come to him?"
He said, "From childhood.
Often it has cast 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."
Jesus said to him, "If you can believe, all things are possible to him who believes."
Immediately the father of the child cried out with tears, "I believe. Help my unbelief!"
When Jesus saw that a multitude came running together,
he rebuked the unclean spirit, saying to him,
"You mute and deaf spirit, I command you, come out of him
, and never enter him again!"
Having cried out, and convulsed greatly, it came out of him.
The boy became like one dead; so much that most of them said, "He is dead."
But Jesus took him by the hand, and raised him up; and he arose.
When he had come into the house, his disciples asked him privately,
"Why couldn’t we cast it out?"
He said to them,
"This kind can come out by nothing, except by prayer and fasting."
What does this reveal about Jesus?
He can heal regardless of the quality of one's faith. But is irritated at unbelief that questions his ability to do these.
In contrast to the Healing of the Gentile Woman's Daughter in which Jesus commended her faith as great even though a Gentile, here he shows his irritation at the characteristic faithlessness of the Jews.
Matthew's account implies the blame is to be place on the unbelief of the disciples, while Marks' account on the unbelief of the father. Both were to blame, not to mention the Jewish society at the time which promoted such unbelief.
But the most interesting expression here is the statement made by the father: "I do believe; help me overcome my unbelief!" This man had a faith, but it was not of a quality acceptable to God to save his son from the demon. We can see this unbelief subtly in his statement, "But if you can do anything, take pity on us and help us." Jesus picked up on this. Paul writes of the quality of Abraham's faith: "Yet he did not waver through unbelief regarding the promise of God, but was strengthened in his faith and gave glory to God, being fully persuaded that God had power to do what he had promised." Rom 4:20,21 And "faith is being sure of what we hope for and certain of what we do not see." Heb 11:1 Which was not the sort of faith the father expressed. Unlike some other miracles previously, he was not questioning Jesus' willingness, but rather his actual ability. This was not acceptable.
So also there are Christians today who while saying they believe do not yet have a quality of faith acceptable to God for their salvation. Such should cry out as the father here, "I do believe; help me overcome my unbelief!" But saving faith is a confident and enduring faith. "We have come to share in Christ if we hold firmly till the end the confidence we had at first." Hebrews 3:14
Consider also Jesus response to his disciples as to why they couldn't drive out the demon. In Matthew he says it is because of their unbelief (little faith). But in Mark he says that this kind could only come out by prayer. If we combine these two we might conclude that the unbelief of the disciples also left them prayerless. If they encountered a barrier it seems they didn't think of praying about it. So also we might use the quality of our prayer life as a measure of the quality of our faith.