The disciples could not drive the demon out of the boy because of the littleness of their faith. The comparison of the mustard seed: if one's faith is even very little, if it is true faith then whatever needs to be done can be done. Genuine faith is not so much quantity as it is quality. The disciples confidence may have outweighed their trust. Little faith may be little in strength because trust and confidence is somewhere else. Some people's self generated 'faiths' fail because they actually believe in what they BELIEVE God will do, rather than simply trusting God (and His being wise in whatever answer He gives, yes or no).