Hereby, Malachi introduces the doctrine of Elias. Joseph Smith taught, "The spirit of Elias is to prepare the way for a greater revelation of God, which is the Priesthood of Elias, or the Priesthood that Aaron was ordained unto. And when God sends a man into the world to prepare for a greater work, holding the keys of the power of Elias, it was called the doctrine of Elias, even from the early ages of the world. John's mission was limited to preaching and baptizing; but what he did was legal; and when Jesus Christ came to any of John's disciples, He baptized them with fire and the Holy Ghost." (Teachings of the Prophet Joseph Smith, pp. 335-6)
Mark also clearly taught that this messenger was John the Baptist, As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare thy way before thee. The voice of one crying in the wilderness Prepare ye the way of the Lord, make his paths straight. John did baptize in the wilderness, and preach the baptism of repentance for the remission of sins (Mark 1:2-4).
One may fairly ask, "If John was the Elias of Christ's First Coming, then who was the Elias of Christ's Second Coming? Was it John the Baptist? Was it Joseph Smith?" The answer is found in the D&C, I have sent mine everlasting covenant into the world, to be a light to the world…and to be a messenger before my face to prepare the way before me (DC 45:9). This is according to the restorative functions of the doctrine of Elias, for there were many angels who brought their keys back to the earth to establish the everlasting covenant in preparation for the Lord's Second Coming. Bruce R. McConkie said, "Who is the promised Elias who was to come and restore all things?…Was it one man? Certainly not. Many angelic ministrants have been sent from the courts of glory to confer keys and powers, to commit their dispensations and glories again to men on earth…it is apparent that no one messenger has carried the whole burden of the restoration." (Mormon Doctrine, p. 221)