Matthew 17:19-21 meaning

The parallel gospel account of Matthew 17:19-21 is found in Mark 9:28-29.

After Jesus cast the demon out of the boy, the disciples came to their Master in order to privately discuss what had just taken place (Matthew 17:14-18). The disciples could mean the Twelve, Jesus' larger number of followers, or only the three who were with Jesus at His transfiguration—Peter, James, and John as they were returning to gather in Galilee (Matthew 17:22).

Matthew emphasizes that the disciples came to Jesus privately (v 19) to show that this conversation took place between them and Jesus alone. Mark said this conversation took place in a house (Mark 9:28). They may have been embarrassed by their failure to cast out the demon but probably more so from Jesus' sigh: "You unbelieving and perverted generation, how long shall I be with you? How long shall I put up with you? Bring him here to Me" (Matthew 17:17).

They asked Him, Why could we not drive the demon out? (v 19). They were asking why they were unable to heal the boy, while Jesus was able. It seems that the disciples were trying to learn from their mistakes. They desired to please Jesus and live up to the Messiah's expectations of them. They wanted to be great, but probably felt inadequate for the tasks of the kingdom—especially after such a public failure.

Jesus answered them directly and truthfully. Because of the littleness of your faith, He told them (v 20). The Greek word that is translated as the littleness of your faith (v 20) is "apistia." More than littleness of faith (like "oligopista"), "apistia" actually means "unbelief." Jesus was not informing His disciples that they had a little faith, but that they had no faith in His power to do this work.

They did not have the faith required to accomplish what God had empowered them to do. The disciples were not relying on God's power to do His kingdom work, and therefore they were unable to do His kingdom work in their own strength. Their strength, power, and abilities were insufficient to do and become the kind of disciples Jesus was calling them to become. But God's power that was available to them by faith was more than enough for them if they trusted in Him.

Having told His disciples why they were unable to drive out the demon (because of their "apistia"), Jesus now proceeded to teach them what they needed to be able to do so. What they needed was faith:

For truly I say to you, if you have faith the size of a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move (v 20).

A mustard seed is a tiny seed, hardly bigger than the size of a grain of sand. But this tiny amount of faith is enough, according to Jesus, to command a mountain to move from here to there (v 20). If you had any faith at all you could do this, Jesus told them. If they had faith in God to do the things that He required of them, Jesus promised them that nothing will be impossible to you (v 20).

God does not desire that we prove anything to Him. God desires that we trust Him. He is interested in our relationship with Him that is based on love and trust. The two primary questions of a believer's relationship with God, are: Do we love God? And do we trust Him? God wishes to partner with us to accomplish great things through us in His mighty power. And in this life, it requires faith for us to partner with Him. When we are unbelieving we can accomplish nothing that will last. When we have faith nothing is impossible and God is greatly pleased.

In Matthew 19 and John 15, Jesus said something similar, that all things are possible through faith:

"And looking at them Jesus said to them, 'With people this is impossible, but with God all things are possible.'"
(Matthew 19:26)

"'I am the vine, you are the branches; he who abides in Me and I in him, he bears much fruit, for apart from Me you can do nothing…If you abide in Me, and My words abide in you, ask whatever you wish, and it will be done for you. My Father is glorified by this, that you bear much fruit, and so prove to be My disciples.'"
(John 15:5, 7-8)

After healing the boy, and saying these things about the importance and power of faith, Jesus says an interesting line about the specific situation the disciples just encountered. But this kind does not go out except by prayer and fasting (v 21).

It seems that what Jesus meant by this kind referred to a particular type or class of demon. Jesus said this kind only goes out by prayer and fasting (v 21). Prayer is communicating with God. Fasting is a discipline of denying a regular good (like eating) for the sake of spiritual growth. Fasting is a means of abstaining from finite goods, such food or pleasure, for the purpose of becoming less dependent upon those goods and more dependent on God. Jesus does not specify who must do the prayer and fasting, but in context it would seem to apply to anyone wishing to see the spirit cast out.

When done correctly, both prayer and fasting are spiritual activities that bring a person into a closer, dependent relationship with God. Prayer and fasting can greatly foster a heart of faith. Perhaps that is why Jesus said this can only be done except by prayer and fasting (v 21). But it could also be that Jesus chastised the disciples because they quit too easily. When they could not cast out the demon, they did not persist in prayer and fasting.

The implication here seems to be that there are various kinds of demonic spirits, and that Jesus knew how to engage with them appropriately. In this case, since Jesus asked how long the boy had been affected, it could be that the differentiation is the duration of the demonic spirit's engagement with the boy. However, since Jesus referred to the spirit as a deaf and mute spirit it could also be that the particular manifestation of the impact on the boy instructed Jesus regarding its "kind." Jesus does not provide instruction on how to discern "kinds." The Bible encourages us to resist Satan and his agents, not to study them (James 4:7). The primary takeaway in application for us might be that we need to continue the resistance without relenting, including in prayer and fasting.

This miracle is a testament to the spiritual authority of Jesus. Jesus's authority is demonstrated by His making this statement about needed prayer and fasting, when Jesus clearly did not need either to pray or fast, but commanded the spirit, who obeyed immediately.

Mark does not record Jesus as including fasting in this final comment.

"And He said to them, 'This kind cannot come out by anything but prayer.'"
(Mark 9:29)

This could tell us that the primary focus is on praying without giving up. The addition of fasting would be a means to intensify and add endurance to the prayer.