Elijah and John
Mark 9:11-12
The disciples were thinking about Malachi 4:5-6. Those verses say that Elijah would return. He would prepare people because the Lord was coming to them.
Jesus showed the disciples that those verses were about John the Baptist. People were cruel to John. And Herod killed John. Jesus explained that people would also be cruel to Jesus himself. The Bible says that these things must happen.