Matt. 11:13 For all the prophets and the law prophesied until John.
14 And if ye will receive it, this is Elias, which was for to come.
Christ is testifying of the Divine calling of John the Baptist. The title Elias mean Forerunner. It was Johns Mission to go before Jesus to prepare his way.
Luke 7:28 There is not a greater Prophet than John the Baptise. Joseph Smith gave 3 reasons why John was the greatest Prophet.
1. He was entrusted with the diine mission to prepare the way before the face of the Lord.
2. He was entrusted with the important mission, and it was required at his hands to baptize the Son or Man.
3. John at the time was the only legal administrator in the affairs of the kingdom there was then on the earth. And holding the keys of power, the Jews had to obey his instructions or be damned, by their own law: and Christ himself fulfilled all rightousness in becoming obedient to the law which he had given to Moses on the mount, and thereby magnified it and made it honorablde, instead of destroying it. The son of Zacharias, (John the Baptist) wrested the keys, the kingdom, the power, the glory from the Jews, by the holy anointing and decree of heaven, and these three reasons constitute him the greatest prophet born to woman
In Mal. it says and I will sent to you Elijah the Prophet before the Great and Dreadful day of the Lord.
This is speaking of Christ Second coming. Elijah appeared to Joseph Smith in the Kirkland Temple in Kirkland Ohio to restore Keys pertaining to work for the Dead.