Luke 17:6 / Matthew 17:20 - Mustard Seed Faith

What does it mean when Jesus says faith the size of a mustard seed can move mountains and uproot trees?

There are two places in Scripture where Jesus uses the mustard seed to describe faith.

The first is in Matthew 17:20. Jesus and three of his disciples have just come down from the mountain where the transfiguration occurred, likely near Caesarea Philippi. They return to a crowd, and a man is complaining that the disciples couldn't drive out a demon from his son. Later, privately, the disciples ask why they failed. Jesus replies:

"Because of your little 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; and nothing will be impossible to you."
Matthew 17:20 LSB

The second is in Luke 17:6. This is a different moment entirely, a different location, likely a different time. (Compare Luke 13:22, Jesus is on a longer journey.) The disciples have just asked the Lord to increase their faith, and He responds:

"And the Lord said, 'If you had faith like a mustard seed, you would say to this mulberry tree, "Be uprooted and be planted in the sea"; and it would obey you.'"
Luke 17:6 LSB

Luke didn't "change" Matthew's mountain into a tree. These are two separate conversations. In Matthew, Jesus may even be gesturing at the mountain in front of them, a physical, looming object used as a prop in the illustration. Luke's passage happens later, in a different place entirely. Two moments. Same seed. Same point.

The consistency of the image is exactly what makes it worth studying.

The Seed

A mustard seed is about 2.5 millimeters in diameter. But under the right conditions, this seed grows into a tree anywhere from 6 to 30 feet tall, depending on the species and climate. In native climates like Iran, dry and sandy, it can get there in 80 to 95 days.

Jesus wasn't being arbitrary when He chose this image. The mustard seed was well known in the ancient Middle East precisely because of this contrast: the smallest seed producing one of the largest plants. He'd already used it in the parable of the mustard seed (Matthew 13:31-32), describing the kingdom of God beginning tiny and growing to encompass the entire world.

The image was already loaded. When He returned to it in Matthew 17 and Luke 17, everyone listening understood: small beginnings are not disqualifying.

Roots

The mustard plant grows as deep as it grows tall.

Its taproot system is a thick central root that typically extends 1 to 3 feet beneath the surface, and in dry conditions can stretch up to 5 feet deep to find underground moisture. Around that taproot spreads a dense network of fibrous roots pulling water and nutrients from the surface.

This is why the mustard tree is practically immovable. It isn't just big above the ground, it's anchored. The same drought conditions that would wilt other plants push the mustard tree deeper.

So when Jesus says faith like a mustard seed can uproot a mulberry tree and throw it into the sea, the contrast is deliberate. The mulberry tree was known in the ancient world for its own notoriously stubborn root system. To uproot it was, in human terms, impossible. What Jesus is describing isn't a feat of human effort. It's the power of God working through true faith.

What is Faith?

A common misreading of these passages is that Jesus is grading faith on a curve. He is not saying even a little belief counts. That's not what's happening here.

The disciples in Matthew 17 had been given the authority to drive out demons (Matthew 10:1). They had seen miracles. They believed Jesus was the Messiah. And yet they failed. Jesus says it's because of their little faith, not the absence of it, but the quality of it.

Scripture gives us a precise definition:

"Now faith is the assurance of things hoped for, the conviction of things not seen."
Hebrews 11:1 LSB

Real faith is not a self-generated feeling of confidence. Scripture is clear that faith itself is a gift:

"For by grace you have been saved through faith; and this is not of yourselves, it is the gift of God; not as a result of works, so that no one may boast."
Ephesians 2:8-9 LSB

And without it, it is impossible to please God (Hebrews 11:6).

The mustard seed isn't a small amount of the same substance as doubt. It's a different kind of thing entirely, a genuine, God-given trust in the God who moves mountains. That's why even the smallest amount of it is powerful beyond measure. It isn't about the size of the faith. It's about who the faith is in.

Just as the mustard seed contains within its 2.5 millimeters everything it needs to become a 30-foot tree, not because of how it feels about growing, but because of what God put inside it, true faith carries the power of the God who gives it.

The Impossible Standard

So why mountains? Why mulberry trees? Why use hyperbole that no one should take literally?

Because the disciples weren't asking about geography. They were asking about the hardest things, about spiritual warfare (Matthew 17), about forgiving people who wrong you repeatedly (Luke 17, just verses earlier). These are the mountains. These are the deeply rooted trees.

Jesus isn't promising a magic formula. He's making a statement about God's omnipotence available to those He has entrusted with faith. The standard is not that you manufacture enough belief. The standard is that you walk in genuine dependence on God rather than in your own strength.

The disciples asked Jesus to increase their faith (Luke 17:5). His answer is telling: He doesn't give them a growth strategy. He points them back to the seed. You don't need more. You need what the seed already is, genuine, rooted, and growing.

Faith isn't measured by size. It's measured by source.

Luke 17:6 / Matthew 17:14-20

Inspiration

This article started with a picture I couldn't get out of my head; the mustard seed root system and how large it actually is. I kept thinking about the disconnect: something 2.5 millimeters wide producing roots that go 5 feet deep. I needed to figure out what that meant, and whether there was a real connection to what Jesus was saying about faith. Turns out there was.

Sources

• From Small Seed to Mighty Tree - PCG Church (seed size, height range, Iran climate, 80-95 day growth)
• Mustard as a Cover Crop - MSU Extension (root depth 1-3 ft, up to 5 ft in dry conditions)
• How deep rooted do mustard seeds get? - r/GardeningUK
• Firefly protein enables visualization of roots in soil - Carnegie Institution for science
• What does it mean to have mustard seed faith? - Got Questions