What does Matthew 17:10 mean?
Peter, James, and John are trying to figure out how what they have just witnessed on the mountain fits with what Jesus has said about rising from the dead.On the one hand, they have just seen with their own eyes Elijah, from heaven, standing on the earth and talking to Jesus. If that was the return of Elijah, though, wasn't he supposed to come before the Messiah? Jesus, who they now understood to be the Messiah, had already come. They ask Jesus why the scribes say that Elijah must come first before the Messiah.
Jesus will confirm that the scribes' insistence that Elijah must come first was right. It was based on the prophecy found in Malachi 4:5–6: "Behold, I will send you Elijah the prophet before the great and awesome day of the LORD comes."