14 And when they had come to the multitude, a man came to Him, kneeling down to Him and saying,
15 “Lord, have mercy on my son, for he is an epileptic and suffers severely; for he often falls into the fire and often into the water.
16 So I brought him to Your disciples, but they could not cure him.”
17 Then Jesus answered and said, “O faithless and perverse generation, how long shall I be with you? How long shall I bear with you? Bring him here to Me.”
18 And Jesus rebuked the demon, and it came out of him; and the child was cured from that very hour.
19 Then the disciples came to Jesus privately and said, “Why could we not cast it out?”
20 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.
21 However, this kind does not go out except by prayer and fasting.”
epileptic (v.15) = the Greek word means “lunatic”
disciples (v.16) — the nine not on the mountain with Jesus
The disciples had been given authority to cast out demons in Matthew 10:8.
Christ responded “O unbelieving generation” (Mark 9:19). He addressed this to the father and to the crowd and to the teachers of the law rather than to the disciples who had been unable to heal the man. This son evidently had been brought as a test of the authority that belonged to Christ and as a test of the disciples as His representatives. The nation to whom Christ had refused to give a sign was still seeking a sign. They would not believe until they had been convinced to their satisfaction that Jesus was who He claimed to be. This confrontation was designed to provide some additional proof by which they might be persuaded; hence, Christ referred to that group as an unbelieving generation, for they had seen a multitude of signs previously and had not believed. — Pentecost, page 260.
What Christ did for the demon-possessed boy was a revelation of what He could and would do for the nation Israel if they would put faith in Him. If they would turn to Him and seek deliverance, He would grant deliverance in response to their faith. Their deliverance did not depend on His ability to deliver but rather on their faith in His person. — Pentecost, page 261.
unbelief (v.20) = little faith
Verse 21 isn’t in the best manuscripts. This account is also found in Luke 9:37-43 and Mark 9:14-29. Verse 21 may have been brought over from Mark 9:29, where it is used in the same context.
Prayer is essentially an attitude of utter dependence on God. These men seem to have been trusting their previous experiences or trusting the authority that had been conferred on them instead of depending on Christ and trusting Him to perform the miracle through Him. — Pentecost, page 261.