While Jesus took Peter, James and John to a mountain to reveal his glory, a man brings his demon possesses son to the rest of the disciples. The disciples fail, so the man approaches Jesus and Jesus easily casts it out of the boy. But why couldn’t the disciples cast out the demon?
Jesus rebuked the demon, and it immediately came out of the child, and he was healed (17:18). The miracle itself is described only briefly. “Rebuked” (ἐπιτιμάω) is the same word Peter used in 16:22 reacted to Jesus’s first prediction of his death as well as Jesus’s response to Peter. Jesus rebuked the sea in Matthew 8:26.
The ESV says Jesus rebuked the demon, the Greek has a pronoun so “Jesus rebuked him/it” and then the demon came out of him (the boy). Potentially the first pronoun could refer to the father or the son, since Jesus does. Not rebuke demons elsewhere in Matthew, only people (Nolland, Matthew, 713).
After he heals the boy, the disciples privately ask Jesus why they could not cast out the demon (v. 19-20). Why? Because of your little faith! That “little faith” (ὀλιγοπιστία) has been a problem throughout the gospel. The disciples were terrified when Jesus walked on the water and Jesus specifically rebuked Peter for is “little faith” (Matt 14:31). Perhaps his lack of faith was on display when he rebuked Jesus (16:23).
The question reflects the disciples’ confusion. They had cast out demons before, now they cannot. Perhaps they tried various tactics to cast out the demon and failed. Second Temple Judaism thought demons could be manipulated by certain rituals. The Testament of Solomon is an example of this. If the exorcist recognized the demon either by name or by the way it effected the person, then they would know the right ritual to use to cast the demon out. Many commentaries will point out the Jewish belief that certain prayers could ward off evil, such as reciting the Shema or Psalm 3 or Psalm 91.
The disciples may have thought the authority given to them to cast out demons in Matthew 10:1 was their authority. Maybe they did not try to cast out this demon “in the name of Jesus.”
Jesus chides his disciples by saying “If you had the faith of a mustard seed you could move a mountain.” This is one of the most memorable sayings of Jesus, but also frequently misused. This saying appears in Matthew 21:21 in a slightly different form, and in two other contexts in Mark 11:23; Luke 17:6; cf., Gos. Thom. 48, 106.
The Gospel of Thomas 48 Jesus said, “If two make peace with each other in this one house, they will say to the mountain, ‘Move away,’ and it will move away.
The Gospel of Thomas 106 Jesus said, “When you make the two one, you will become the sons of man, and when you say, ‘Mountain, move away,’ it will move away.’”
Mustard seed is the smallest possible amount of something. Jesus has already compared the kingdom of heaven to a mustard seed, but this use of the image is different. Jesus says if you even had the tiniest shred of faith, you could have healed the demon oppressed boy. This does not refer to self-confidence, faith in oneself, etc. It is faith in God as he is now working through Jesus. “It is not the amount of faith that is in question, but rather its focus” (Wilkins, Matthew, 597). Maybe this was the problem: The disciples were trying to cast out the demon in their own authority rather than by the authority given to them by Jesus: “in Jesus’s name.”
“To move mountains” is proverbial for doing the impossible, so the opposite of tiny amount of faith. This is hyperbole, any “literal interpretation is clearly ludicrous” (Davies and Allison, Matthew, 2:726). In Isaiah 54:10 the mountains may depart, but God’s love will not. Josephus uses a similar phrase when the Lord is encouraging Moses: “for even these mountains, if God so please, may be made plain ground for you, and the sea become dry land.” (Ant. 2:333). In Testament of Solomon 23:1, Solomon interrogates the demon Ephippas who claims, “I am able to move mountains, to carry houses from one place to another, and to overthrow kings.”
Nothing would be impossible? Wilkins comments that faith is not a substance, the more you have the more power you have, or a kind of magic to be manipulated, but confidence that “we can do what God calls us to do” (Matthew, 597). This is important with respect to the application of this saying. Jesus is not saying if you had more faith, you could pray and impossible things would happen (you get suddenly rich, someone is miraculously healed, etc.) The disciples were given authority over demons, but they failed because they were trying to manipulate God like an ancient magician.
Having shown his power to his closest disciples on the mountain, Jesus demonstrated his authority once again over demons. He continues to teach his disciples so that they fully understand who he is, leading to the second of three predictions of his coming death and resurrection.
Final note: Where is Matthew 17:21? Most modern translations omit this verse. The final line “But this kind never comes out except by prayer and fasting” was added in the copying process based on the story in Mark, offering another explanation for the disciples’ failure. The verse is missing in earlier manuscripts, א* B Θ, etc. It was added to Sinaticus by the second corrector and is found in the Byzantine tradition (hence its appearance int he KJV Bible).