Matthew 17:20
“Because you have so little faith,” He answered. “For truly I tell you, if you have faith the size of a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”
What it says
Jesus says their faith is too small and compares mustard-seed faith to a mountain moved by command.
What it is doing
Defines faith as effective trust rather than mere size.
In context
19Afterward the disciples came to Jesus privately and asked, “Why couldn’t we drive it out?” 20“Because you have so little faith,” He answered. “For truly I tell you, if you have faith the size of a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”