Matthew 17:20 — Unbelief or Little Faith? Why the Disciples Could Not Cast It Out

“And Jesus said unto them, Because of your unbelief.” — Matthew 17:20

The disciples had tried and failed to cast out a devil. When they ask why, the received text answers unbelief. The critical text answers little faith. The difference is one Greek word: ἀπιστία against ὀλιγοπιστία.

Both rebuke defective trust. They do not, however, sound the same. Little faith is weak; unbelief is a refusal or failure to believe.

“Little faith” is a favorite Matthean expression. Jesus uses related forms when the disciples fear the storm, worry about clothing, and misunderstand the leaven. The rarer noun here may therefore look like Matthew’s own word. Critics also note “faithless generation” in verse 17 and argue that a copyist changed little faith to unbelief under its influence.

The received reading has its own contextual strength. The disciples did not merely bring a small amount of trust to a task that required more spiritual force. At the decisive point they failed to rely on Christ’s granted authority. Their unbelief need not mean that they possessed no saving faith. Scripture can rebuke belief mixed with unbelief: “Lord, I believe; help thou mine unbelief.”

The following image supports the sharper word. Faith as a grain of mustard seed moves a mountain. Jesus does not measure a large faith against a small one as though power rose from spiritual quantity. Even tiny true faith lays hold of a mighty God. Their failure lay in unbelief, not in the physical size of an inner substance.

Faith is not a lever

“Nothing shall be impossible unto you” is not a blank check for private ambition. It belongs to the apostolic work Christ assigned and to dependence on his power. The mountain figure names an obstacle no human strength can move.

This keeps the received reading from becoming a weapon against troubled Christians. Christ is not saying that every unanswered prayer proves a person unconverted. He is training appointed servants whose act had become detached from prayer, fasting, and present trust.

The early support for “little faith” and Matthew’s fondness for the term make it a serious rival. The received “unbelief” nevertheless fits the immediate “faithless generation,” the mustard-seed contrast, and the church’s continuous text. It also prevents a common mistake: the answer to weak ministry is not to admire the size of our faith but to turn from unbelief to the Lord who acts.

Mustard seed is small, but living. Christ’s image does not invite disciples to measure spiritual confidence until it reaches mountain-moving volume. It contrasts genuine reliance upon God with the unbelief that attempts ministry from remembered authority while neglecting present dependence.

Mark’s account includes the father’s cry, “Lord, I believe; help thou mine unbelief.” That mixed confession shows how Scripture can use unbelief of a real believer’s remaining failure. The received Matthew reading therefore need not declare the apostles unregenerate.

The variant also warns against diagnosing every failed prayer. The disciples had received a particular commission and asked about a particular exorcism. Christ’s answer trains them; it does not grant observers a universal right to blame the sufferer or petitioner whenever a desired act does not occur.

Sources and further reading

• Matthew 6:30; 8:26; 14:31; 16:8; 17:14–21; Mark 9:14–29.
• Bruce M. Metzger, A Textual Commentary on the Greek New Testament, note on Matthew 17:20.
• John William Burgon and Edward Miller, The Traditional Text of the Holy Gospels.