When they came to the crowd, a man approached Jesus and knelt before him.
“Lord, have mercy on my son,” he said. “He has seizures and is suffering greatly. He often falls into the fire or into the water.
I brought him to your disciples, but they could not heal him.”
“O unbelieving and perverse generation,” Jesus replied, “how long shall I stay with you? How long shall I put up with you? Bring the boy here to me.”
Jesus rebuked the demon, and it came out of the boy, and he was healed from that moment.
Then the disciples came to Jesus in private and asked, “Why couldn’t we drive it out?”
He replied, “Because you have so little faith. I tell you the truth, if you have faith as small as a mustard seed, you can say to this mountain, ‘Move from here to there’ and it will move. Nothing will be impossible for you.”
[But this kind does not go out except by prayer and fasting.]

GNT … If you have faith no larger than a mustard seed, you can tell this mountain to move from here to there and it would. Anything can be possible for you… Mat 18:18 …so I tell all of you: what you prohibit on earth will be prohibited in heaven and what you permit on earth will be permitted in heaven’

The mustard seed is not the world’s smallest seed, but at the time of Jesus it was the smallest planted seed that grew to an astonishing 3 to 4 meters (2 ft+). Comparing an amount of Faith to a small seed, that can do great things, like the impossible of moving a mountain, is a good way to teach us how! But just like planting a seed, we need to plant ourselves in the right place (choose a Good & faith filled Church), make sure the soil is rich and fertile (get Biblical and evenly spread teaching), make sure we get the right combination of food, (teaching of Biblical doctrine based around the one great doctrine… God’s Love).