Matt 17:14–20 — Faith Like a Mustard Seed

The miracles Jesus performed were not merely displays of power. They demonstrated his identity as the Messiah who saves his people from sin and brings the blessings of God’s kingdom. Throughout Matthew’s Gospel, especially in chapters 8–9, these works have affirmed Christ’s messianic authority. The disciples have understood their significance and confessed Jesus as the Messiah, the Son of the living God. They know that the divine Son has become man to save his people from their sins.

Matthew 17:14–20, however, is not simply another account of Jesus casting out a demon. Jesus does rebuke the demon, and the boy is instantly healed, but the central concern is the disciples’ failure. Jesus had granted them authority to cast out demons, yet they proved unable to cast out this particularly stubborn one. Their question—“Why could we not cast it out?”—gives Jesus an opportunity to teach them about faith. This is faith like a mustard seed: lessons from a failed exorcism.

The Disciples’ Inability

As Jesus, Peter, James, and John descend from the Mount of Transfiguration, they encounter a crowd. A desperate father kneels before Jesus and pleads for mercy:

Lord, have mercy on my son, for he has seizures and he suffers terribly. For often he falls into the fire, and often into the water. And I brought him to your disciples, and they could not heal him.

The boy is tormented by a cruel demon that causes seizures, takes control of his body, and throws him into fire or water to destroy him. The other nine disciples, who had not accompanied Jesus up the mountain, attempted to heal the boy but failed.

Their failure is striking because Jesus had specifically granted them authority over demons. Matthew 10:1 records that he called the twelve and “gave them authority over unclean spirits, to cast them out, and to heal every disease and every affliction.” On this occasion, however, even strenuous effort accomplished nothing. The demon would not yield to their authority. The father therefore comes to Jesus with the painful report: “I brought him to your disciples, and they could not heal him.”

Christ’s Complete Ability

The disciples’ inability stands in sharp contrast to Jesus’ complete ability:

O faithless and twisted generation, how long am I to be with you? How long am I to bear with you? Bring him here to me.

Jesus responds to the father’s plea by rebuking the disciples. His language is unusually forceful. On another occasion, when Peter began to sink while walking on the water, Jesus gently asked, “O you of little faith, why did you doubt?” Here the rebuke is much sharper and identifies a profound failure of faith.

The expression “faithless and twisted generation” comes from Deuteronomy 32:5. There Moses looks ahead to Israel’s history under the law and foretells the nation’s apostasy. By echoing Moses’ words, Jesus emphasizes the severity of the disciples’ collapse. His frustration is palpable: “How long am I to be with you? How long am I to bear with you?”

Why is Jesus so severe? The nine disciples had tried to cast out the demon in their own authority and power. They relied upon themselves rather than Christ. Jesus had indeed authorized them to cast out demons, but their authority was always delegated. Power over Satan belonged to Christ, never to the disciples independently of him.

Jesus makes that reality unmistakable. He commands that the boy be brought to him, rebukes the demon, and the demon immediately departs. The boy is healed at once. The disciples are completely unable; Christ is completely able.

The Disciples’ Question

Afterward, the disciples approach Jesus privately and ask, “Why could we not cast it out?” Their wording shows that they still do not fully understand. They ask why they could not do it, when the point is that only Christ possesses the power. Even when they cast out demons, they do so only through authority he delegates to them.

Faith looks to Christ from the position of our weakness and inability. It confesses that we are utterly bankrupt and that Christ alone possesses the infinite riches of saving grace. Jesus therefore answers, “Because of your little faith.” Their failure arose from their lack of confidence in him.

Jesus continues:

Truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, “Move from here to there,” and it will move, and nothing will be impossible for you.

On the surface, this sounds like a blank-check promise of unlimited power. Does it support the “word of faith” or “name it and claim it” teaching that Christians may obtain anything if they believe strongly enough? It does not. If Jesus intended his words literally and without qualification, we would expect examples in Acts of apostles physically relocating mountains. We find none.

Calvin rightly identifies the statement as hyperbole. He also explains that the Lord does not promise to grant whatever random wish enters our minds. Rather, “those in whom faith reigns do not desire every thing indiscriminately, but only that which the Lord promises to give.” Faith is directed toward God’s promises. Verse 20 is not a promise of magical power but a call to trust Christ concerning what he has pledged to accomplish.

The Promise Behind the Command

Jesus’ promise looks both backward and forward. It looks backward to the authority he had already given the disciples to proclaim the kingdom and cast out demons in his name. He had promised to empower them, but in this encounter they forgot both the promise and its source.

The saying also looks forward to their apostolic mission after Jesus’ resurrection. The reference is not to mountains in general but to “this mountain.” Mountains carry important symbolism throughout Scripture and especially in Matthew. Jesus delivers the Sermon on the Mount as the new Moses. He is transfigured on a high mountain. At the end of the Gospel, the eleven gather on the mountain Jesus appointed, where the risen Christ declares, “All authority in heaven and on earth has been given to me,” and commissions them to make disciples of all nations.

In Matthew 17, however, “this mountain” must be removed to make way for the true mountain of God. It represents the Jerusalem leadership and temple system that holds Israel in legal bondage and stands opposed to Jesus. Although its removal appears impossible to the disciples, it will be taken out of the way. Through the apostles’ preaching, Jesus will build his eschatological temple upon the mountain of God.

This interpretation accords with Jesus’ promise to Peter in Matthew 16:18–19:

I tell you, you are Peter, and on this rock I will build my church, and the gates of hell shall not prevail against it. I will give you the keys of the kingdom of heaven.

The language of building is temple imagery. Christ will build his spiritual temple, the church, and the gates of hell will not prevail against it. He will entrust the keys of the kingdom to his apostles. The promise in Matthew 17:20 therefore concerns their God-given mission, not every personal desire. It is no promise that a flat tire will repair itself if we believe firmly enough. Christ promises to accompany and empower the apostles as he builds his church through their ministry.

The passage concerns a specific failed exorcism and promises made to the apostles, but it also teaches every believer three broader lessons about faith: its precondition, its object, and its effectiveness.

The Precondition of Faith

The disciples failed because they relied upon themselves. They were self-confident when they should have been Christ-confident. Their question—“Why could we not cast it out?”—reveals their assumption that the necessary power should have resided within them.

We often repeat their mistake. We become confident in the face of difficulty, assume that we can manage in our own strength, and then experience failure. Our confidence collapses, and we find ourselves confronting our lack of faith.

The Lord allowed the disciples to fail so that they would become thoroughly convinced of their inability. At times he likewise allows us to stumble. He strips away our self-confidence so that our hope will rest not in ourselves but in Christ alone.

The precondition of faith is therefore the loss of self-confidence and an honest recognition of weakness. That place of failure may feel deeply uncomfortable, but it is a good place to be. When we know our inability, we can call upon the Lord, direct our faith to him, and confess that Christ alone possesses the power we need.

The Object of Faith

True faith is confidence directed toward Christ. The disciples’ fundamental problem was that they had lost sight of him—and, more specifically, of Christ crucified.

The immediate context repeatedly emphasizes Jesus’ coming suffering. In Matthew 16:21, Jesus begins to show his disciples that he must go to Jerusalem, suffer at the hands of the elders, chief priests, and scribes, be killed, and rise on the third day. As he descends the Mount of Transfiguration, he commands Peter, James, and John to tell no one about the vision until the Son of Man has risen from the dead, implying that death must precede resurrection.

Jesus then identifies John the Baptist as the promised Elijah who came and was mistreated. John was beheaded in Matthew 14. Jesus adds, “So also the Son of Man will certainly suffer at their hands.” Immediately after the failed exorcism, Jesus again predicts that the Son of Man will be delivered into human hands, killed, and raised on the third day. The disciples are greatly distressed.

They have confessed Jesus as the Messiah and Son of God, but they struggle to grasp that he is a suffering Messiah. They prefer a glorious conqueror. They want to bypass the cross and reach glory by another route. Casting out a demon would be an impressive display of power, but they want that power apart from dependence upon the crucified Christ.

No wonder they fail. Satan is bound only through the cross. It is through the Messiah’s suffering on behalf of his people that the devil is conquered and brought under Christ’s authority. Faith does not seek spiritual power independently of the Savior’s death; it lays hold of Christ crucified.

Faith’s precondition is the recognition of our inability. Its object is not faith itself, but Christ—and specifically Christ crucified.

The Effectiveness of Faith

The effectiveness of faith is measured not by its size but by its object. Faith may be as tiny as a mustard seed, but if it lays hold of Christ, it is sufficient because Christ is sufficient.

Faith is not an attempt to produce an intense internal feeling of certainty. It is not a heroic deed or a mighty spiritual achievement. It is the humble acknowledgment of our inadequacy and complete reliance upon Christ. Faith itself is empty; all its value lies in the one whom it receives.

Paul joins faith so closely to its object that, in Galatians 3, he can even refer to Christ’s coming as the coming of faith. In explaining the role of the law in redemptive history, Paul says that the law was added because of transgressions until the promised offspring, Christ, should come. He then writes, “Before faith came, we were held captive under the law,” and, “Now that faith has come, we are no longer under a guardian.”

Paul cannot mean that no one had faith before Christ’s incarnation. In the same chapter, he emphasizes that Abraham believed God and that believers today share Abraham’s faith. Abraham lived two thousand years before Jesus, yet his faith was the same in kind as ours.

When Paul says that faith came, he therefore refers to the coming of faith’s object. Christ has come. Paul binds faith and Christ together so closely that he can speak of the object of faith as “faith.” This illustrates the fundamental point: faith in itself is nothing; Christ is everything.

Martin Luther writes in his commentary on Galatians, “Faith is neither law nor work, but an assured confidence which apprehendeth Christ” (Kregel edition, transl. Middleton, p. 218). Faith is not a great work we perform. Like a mustard seed, it is small. It simply apprehends Christ and rests upon him.

The Lord did not abandon the disciples after their failure. He exposed their self-reliance, but he also encouraged them with a promise. Even mustard-seed faith is effective when it depends wholly upon him. He likewise uses our failures to convince us that the power is not ours but belongs to Christ crucified and risen.

Our insufficiency is real, but so is Christ’s all-sufficiency. We do not need impressive faith in our own faith. We need faith that turns away from itself and lays hold of Jesus. Faith itself is nothing. Christ crucified is all.