WHY COULD THE DISCIPLES NOT CAST OUT THE DEVIL IN MATTHEW 17:14-21?
by Shawn Brasseaux
Matthew chapter 17 relays a strange account: “ And when they were come to the multitude, there came to him a certain man, kneeling down to him, and saying,  Lord, have mercy on my son: for he is lunatick, and sore vexed: for ofttimes he falleth into the fire, and oft into the water.  And I brought him to thy disciples, and they could not cure him.  Then Jesus answered and said, O faithless and perverse generation, how long shall I be with you? how long shall I suffer you? bring him hither to me.  And Jesus rebuked the devil; and he departed out of him: and the child was cured from that very hour.
“ Then came the disciples to Jesus apart, and said, Why could not we cast him out?  And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.  Howbeit this kind goeth not out but by prayer and fasting.”
The Lord Jesus accused His disciples of unbelief (verse 20). What exactly did they not believe? They did not believe the divine revelation He had just given them about His coming death! Earlier in the chapter, descending the Mount of Transfiguration, He told them He would die.
“ And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.  And his disciples asked him, saying, Why then say the scribes that Elias must first come?  And Jesus answered and said unto them, Elias truly shall first come, and restore all things.  But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.  Then the disciples understood that he spake unto them of John the Baptist.” The parallel passage, Mark 9:9-13, makes this clearer—especially verses 9-10.
Since the disciples were not operating on faith—trusting what God had just shown them—they lacked His power to perform the miracle. Matthew 17:21 is of particular importance, which is why we need a King James Bible (that contains it). The verse is omitted from modern English versions because their underlying manuscripts omit it: “Howbeit this kind goeth not out but by prayer and fasting.” The prayer and fasting, of course, is speaking to God in light of His Word and then mourning. They are to lament Christ’s soon death.
Matthew 9:14-15 shows us: “ Then came to him the disciples of John, saying, Why do we and the Pharisees fast oft, but thy disciples fast not?  And Jesus said unto them, Can the children of the bridechamber mourn, as long as the bridegroom is with them? but the days will come, when the bridegroom shall be taken from them, and then shall they fast.” Jesus is the Bridegroom, and He will soon be “taken from them” (killed)! This is when they are to mourn, He said.
Let us return to Matthew chapter 17, after Jesus performed the miracle the disciples could not execute. We read the following: “ And while they abode in Galilee, Jesus said unto them, The Son of man shall be betrayed into the hands of men:  And they shall kill him, and the third day he shall be raised again. And they were exceeding sorry.” This time, they understood what He meant. They mourned His approaching death. However, the disciples have lost their power to perform miracles.
These disciples will not regain that power until Pentecost, Acts chapter 2, when the Holy Spirit comes down and indwells them (see John 14:16-20, John 15:26, and John 16:7-16). Luke 24:49 says: “And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be endued with power from on high.” Also Acts 1:8: “But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.”
Acts chapter 2: “ And when the day of Pentecost was fully come, they were all with one accord in one place.  And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting.  And there appeared unto them cloven tongues like as of fire, and it sat upon each of them.  And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance….  And fear came upon every soul: and many wonders and signs were done by the apostles.”
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