What does Matthew 17:13 mean?

Will Elijah the prophet return before the coming of the Messiah? Malachi 4:5–6 says He will. Peter, James, and John have just seen Elijah on the mountain talking to Jesus, apparently from heaven. If Elijah has not yet returned to earth to lead the people to repentance, how can Jesus be the Messiah?

Jesus has told them that the "Elijah" prophesied in the Old Testament has returned but was not recognized by Israel's religious leaders. He had said this before in Matthew 11:14. Now the disciples realize that Jesus is saying John the Baptist fulfilled the prophecy of Elijah's return.

This is not simply a convenient way for Jesus to explain away an inconvenient prophecy. God's intention was always for John the Baptist to fulfill that prophecy. Before John was born, an angel told Zechariah, John's father, that John would "turn many of the children of Israel to the Lord their God, and he will go before him in the spirit of Elijah, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just, to make ready for the Lord a people prepared" (Luke 1:16-17).

Somehow, John the Baptist operated in the spirit and power of Elijah. It's likely some expected Elijah himself to be resurrected and once again minister in Israel—this an idea John himself rejected (John 1:20–21). God's plan was for John the Baptist to do the work of Elijah, in the spirit and power of Elijah, to accomplish God's plan to make the people ready for Jesus the Messiah (Matthew 11:14).