Again, what Jesus was saying about Elijah took the disciples a long time to understand. But, to their credit, they were trying to apply the Scripture (Malachi 4:5) to their present experience.
When they come down and encounter the demon possessed boy, Jesus points out the lack of faith, twice. If lack of faith was a problem for them, how hard it must be for Him to put up with me.
Another statement about Jerusalem. The march to the cross cannot be ignored!