"'If anyone causes one of these little ones - those who believe in me - to stumble, it would be better for them if a large millstone were hung around their neck and they were thrown into the sea.'" NIV translation
The Hebrew word that is used here for stumble is kashal which means to totter or waver. Jesus tells his disciples that it would be better to die a horrible death than to cause a believer to waver in their faith. There has been much confusion about what this means but it simply means causing someone to doubt the sufficiency of Jesus for their salvation. There are many that try to add a certain part of doctrine or tradition to distinguish them as the "true Christians". That is, in fact, why there are so many denominations and a lack of unity in the church as a whole. Jesus is speaking about them and the fact that they cause people to doubt whether they are truly saved.
"'If your hand causes you to stumble, cut it off. It is better for you to enter life maimed than with two hands to go into hell, where the fire never goes out. And if your foot causes you to stumble, cut it off. It is better for you to enter life crippled than to have two feet and be thrown into hell. And if your eye causes you to stumble, pluck it out. It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell, where "the worms that eat them do not die and the fire is not quenched."'" NIV translation
Now, Jesus speaks to the people that are teaching this legalism to others.
He lists parts of the body that can be used for work and says that it is better to not have them than to have them and trust that you can add anything to His sacrifice for your salvation.
The reason that Jesus does this is because He wants us to understand that it is through His grace alone that we are saved.
"'Everyone will be salted with fire. "Salt is good, but if it loses its saltiness, how can you make it salty again? Have salt among yourselves, and be at peace with each other."'" NIV translation
Salt is a preservative and fire speaks of refinement. Jesus is saying that we all will face doubts and testing of our faith but we must hold onto Him and His grace. He also speaks of peace among the believers and that only comes with freedom to have differences.