Matthew 17:14-20 (ESV)
And when they came to the crowd, a man came up to him and, kneeling before him,  said, "Lord, have mercy on my son, for he is an epileptic and he suffers terribly. For often he falls into the fire, and often into the water.  And I brought him to your disciples, and they could not heal him."  And Jesus answered, "O faithless and twisted generation, how long am I to be with you? How long am I to bear with you? Bring him here to me."  And Jesus rebuked him, and the demon came out of him, and the boy was healed instantly.  Then the disciples came to Jesus privately and said, "Why could we not cast it out?"  He said to them, "Because of your little faith. For truly, I say to you, if you have faith like a grain of mustard seed, you will say to this mountain, 'Move from here to there,' and it will move, and nothing will be impossible for you."
Faith like the grain of a mustard seed, this is one of those passages that makes people cringe and fall into despair. It leaves you wondering whether you have faith at all, and faith alone saves. I mean most of us aren’t able to heal sick people, or move mountains, or anything of the like. It should be noted the disciples had cast out demons before. They had been able to do many miraculous things with their faith. Jesus accuses them of not having a faith like the grain of a mustard seed. He does not accuse them of not having faith. nor does he say they aren’t saved.
Bo Giertz talks about different types of faith. Saving faith and miracle making faith. I suppose that is a good explanation, as well as it goes. The idea behind it being don’t despair if you don’t actually move a mountain with your prayer.
It is also true as Paul later explains that different people have different gifts that accompany their faith. Not all are going to speak in tongues, or heal. Not all are going to have the gift I covet most of being good administrators.
I think perhaps the greater take away from this though, is that we all have room to grow. The disciples had faith, they still had room to grow with their faith. We all have that. Salvation is complete, sanctification is complete in baptism. Yet not for the sake of working out our salvation, but for the sake of growing in faith and love and the joy of our salvation we are never done with God’s word, never finished with his sacraments, our faith can always grow this side of glory.