Some people type this question with someone they love in the next room, sick, after months of praying. If that is you, start here: a mountain that is still standing is not, by itself, a sign that your faith is too small. Jesus did say that faith as small as a mustard seed can move a mountain, and he meant it. The measure he gives is a seed, the smallest amount, pointed at the God who does the moving.
One more thing first, because this verse gets used on families. In Matthew, the words “Because of your unbelief” were said in private to the disciples, the men who had tried to heal a sick boy and could not. They were said to them, and the father who brought the boy heard something else. The day it happened shows what.
A father in the crowd
Mark tells that day from a father’s side. A crowd has gathered, scribes are questioning the disciples, and a father has brought his son, who has been sick since he was a child. “I spake to thy disciples that they should cast him out,” he says, “and they could not.” Jesus answers him, and the rebuke takes in a whole generation, “O faithless generation, how long shall I be with you? how long shall I suffer you?”, and then it turns into an invitation: “bring him unto me.”
The boy has been thrown into fire and into water. The father’s request begins with an “if” and has a small word in it twice: “if thou canst do any thing, have compassion on us, and help us.” He counts himself inside the need.
If thou canst
Jesus said unto him, If thou canst believe, all things are possible to him that believeth. And straightway the father of the child cried out, and said with tears, Lord, I believe; help thou mine unbelief.
Mark 9:23-24 (KJV)
Jesus hands the “if” back to him. The father answers with both halves of the truth at once. He believes, and his belief has a hole in it, and he says so out loud, in tears, while asking anyway. He does not wait until the second half is mended. Three verses later Jesus “took him by the hand, and lifted him up; and he arose.”
That prayer is the one to borrow when your own faith feels too thin to count. You can bring what you are carrying with the faith you have, and tell God plainly where it is thin.
The question they asked in private
Afterward, away from the crowd, the disciples ask what went wrong, and Jesus says the line about the mustard seed.
Then came the disciples to Jesus apart, and said, Why could not we cast him out? And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.
Matthew 17:19-20 (KJV)
Jesus does name unbelief here, and he names it in the men who tried to heal the boy and could not. Even to them, the measure he gives is the smallest seed; he does not ask them for a larger stock of faith. Matthew goes on, “Howbeit this kind goeth not out but by prayer and fasting,” and Mark gives only that answer. Prayer is how a little faith turns to God. In this story the one who was heard came with a plea, a cry and tears, and Mark mentions no fasting from him.
Luke has the same seed in another conversation, where the apostles ask, “Increase our faith.” Jesus answers a request for more with a seed.
When the mountain does not move
If you are praying for someone who is not well yet, this verse can turn into a sentence passed on yourself: if I had even a seed of faith, it would have moved by now. I know that reading from the inside. I used Matthew 17:20 as a measuring device, and the measurement always came out that whatever had not happened had not happened because of me. Nothing in ask and you shall receive puts the one asking at fault either.
If someone has used this verse on your family, to explain why the one you love is still sick: in Matthew 17 the words “because of your unbelief” went to the men who tried to heal and could not. And in Mark the father who admitted his own unbelief, in tears, is the one whose son Jesus took by the hand.
Mark 11 has the strongest words on this. “Have faith in God,” Jesus says, and then, “whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart… he shall have whatsoever he saith.” Those words begin with God as the one the faith is in. And on the night before he died, Jesus himself prayed, “Abba, Father, all things are possible unto thee; take away this cup from me: nevertheless not what I will, but what thou wilt.” The cup did not pass. A mountain that has not moved is not, by itself, a measurement of your faith.
The smallest real faith is enough to bring God anything, and the father in Mark 9 shows what it sounds like. It sounds like belief with a hole in it, said out loud, while asking anyway: “Lord, I believe; help thou mine unbelief.”