Having had that wonderful opportunity to see the Lord Jesus in His glory, at the top of the mountain, Peter, James and John descended with Him to the valley where they joined the other disciples again.
A large crowd had gathered there, also some teachers of the Law who were arguing with them. Among the crowd was a man who had brought his son, possessed by a demon, which caused him symptoms of epilepsy and prevented him from speaking.
In his third trip to Galilee, the Lord Jesus had given His disciples authority to expel unclean spirits and to cure all diseases and illnesses, in a specific mission to announce only to Jews that the Kingdom of Heaven had come (Chapter 10). During that mission they succeeded with these signs (Mark 6:13).
The man had now asked the disciples to expel the spirit from his son. In the absence of the Lord, remembering the powers they received on that occasion, the disciples sought to attend to the request of the man, but were unsuccessful despite their efforts.
Seeing the Lord Jesus, the people were very surprised and went to greet him (Mark 9:15). The man also approached, knelt in reverence before Him, and asked for mercy, explaining the situation. The response from the Lord Jesus has given reason to much controversy since antiquity: what did He mean by the "faithless and perverse generation" who seem to have annoyed Him?
The disciples were never called “perverse” or “unbelievers”, though they still had little faith (verse 20). They still had a lot to learn, and could not yet discern the spirits, this one being of a kind that was more difficult to expel than others (verse 21).
By saying "faithless and perverse generation”, He might be referring to the Jewish people in general (chapter 23:36, 24:34), at that time (chapter 11:16), or only to the scribes present who were arguing with His disciples (chapter 12:39-45, and chapter 41). It seems more likely it was to the Jews of that time, as Matthew does not mention the scribes. Possession by demons had become common, being a result of the apostasy of the Jewish people. They became perverse and incredulous at the time of the coming of the Messiah, and would reject Him to the point of killing him, as the Lord repeated to His disciples following this episode.
The boy's father seemed to doubt a little, saying to the Lord "if You can do anything, have compassion on us and help us", to which He replied "If you can believe, all things are possible to him who believes" (Mark 9:23).
The essential condition for God to work in our lives, and still do miracles, is that we believe in His son, the Lord Jesus Christ. It is not a matter of believing that God can do miracles, or having great faith in His servants doing miracles. It is saving faith evidenced by conversion and surrender to Him.
The poor boy's father, ready to do anything to free him from his oppressor, exclaimed, "Lord, I believe; help my unbelief!" (Mark 9:24). The merciful Lord Jesus did not wait any longer and immediately ordered the spirit to get out of the boy. He did so, not without first giving one more demonstration of his wickedness.
The disciples were still curious to know the reason why they had not been able to expel that demon, and later asked Him to explain it to them once they were alone.
He gave them two reasons:
He said that if they had faith the size of a mustard seed, they could move that mountain – it would be that of the transfiguration! The mustard seed was the smallest that they knew (Mark 4:31) and this was certainly a popular expression to indicate the smallest thing you could imagine. The little faith that moves the high mountain was a parable.
Their faith had no substance, and was inoperative. They still had a lot to learn until they understood the reality of the Messiah, the Saviour, His saving mission and the true character of the mission that they had to perform as His witnesses.
For now, they trusted only on the authority that He had given them earlier, during their mission to preach the Kingdom to the Jews. This mission was over, and they had no competence to continue doing more miracles.
Instead of seeking to expel this demon with the powers they had received, they had to intercede with God, with prayer and fasting, so that He might grant their request. The Lord Jesus said on another occasion that, after He had gone to heaven, the disciples could ask the Father everything in His Name (John 14:13).
After His death and resurrection, Jesus Christ again granted, only to those who among the apostles really believed in His divine person, having accepted his redeeming death, special gifts of healing diseases and expulsion of demons - Judas Iscariot was no longer counted among them (Mark 16:17 -18).
For the third time the Lord Jesus said to His disciples that He was to be delivered into the hands of men to be killed, but He would rise again. Who would hand Him over? Judas Iscariot (chapter 26:14-16), thus fulfilling the prophecy in Jeremiah 18:18 and Zechariah 11:13.
The disciples were filled with sadness, not understanding what His death meant, resurrection coming after, and they were afraid to ask (Mark 9:32). Every believer needs to search the Word of God for answer to his doubts, to increase his faith.
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."
22 Now while they were staying in Galilee, Jesus said to them, "The Son of Man is about to be betrayed into the hands of men,
23 and they will kill Him, and the third day He will be raised up." And they were exceedingly sorrowful.
Gospel of Matthew, chapter 17, verses 13 to 23