Jesus gave some very severe warnings to stress the importance of faith.
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.
A millstone was a heavy stone that was used to grind corn. Jesus is saying that if a person caused someone to lose their faith they would be better off dead.
Jesus then uses a number of images to stress that the disciples must stop anything that is causing them to lose faith. If their hand or feet causes them to sin, they are to cut them off. If their eye causes them to sin, they must pluck it out.
Jesus’ message here is that nothing is more important than gaining entry into the Kingdom of God.