...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.
In The Brothers Karamazov, several characters are discussing faith and whether or not a man can claim to be Christian if he has not faith even as a mustard seed with which one should be able to move mountains. If forced with the decision to be tortured or killed unless you renounce Christ and you cannot cause a mountain to fall on and crush your enemies, you are not a Christian, and therefore are renouncing nothing if you renounce Christ. I'm not sure how clear that is, but let me put it in terms that enlightened me.
In this case, if you have the amount of faith equal to the size of a mustard seed (very small) you should be able to move mountains. I doubt many people are able to do this currently, but at the same time I know many people of such great faith. How can this be? If you read the scripture carefully the Lord says, "if you have faith as a grain of mustard seed." This could very well mean, "if you have faith like a grain of mustard seed has faith." The mustard seed is the smallest seed that yields the biggest tree for its relative size. That mustard seed, though small, can reach a potential far greater than any other seed. If we have that type of faith we, too, can reach a potential far greater than any other, eternal life.
That faith gives men power, and in the case of the scripture should have been the power by which the Lord's disciples attempted to heal the lunatick. But because they did not have that power, that faith (accessible through prayer and fasting) they could not heal the man. Faith is not only belief, it encompasses obedience as well. We can only be tools to do the Lord's work if we are worthy of such and capable to know His will concerning the things over which He has given us dominion.