…continuing…
Elias and the “Restoration of All Things”
This point is supported by the Latter-day Saint and early Christian doctrine of Elias. “Elias” is the Greek form of the Hebrew name “Elijah.” In the last verses of the Old Testament the promise is made: “Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord . . . .” (Malachi 4:5) After Elijah himself appeared before Jesus, Peter, James, and John on the Mount of Transfiguration, Jesus told his disciples to tell no one of the vision until after His resurrection from the dead. But then the Apostles asked:
Why then say the scribes that Elias must first come? And Jesus answered and said unto them, Elias truly shall first come, and restore all things. But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them. Then the disciples understood that he spake unto them of John the Baptist." (Matthew 17:3-13)
What did Jesus mean when He said that John the Baptist was Elias, even though Elijah and Moses had just appeared? The angel Gabriel told John’s father that he would “go before him [Christ] in the spirit and power of Elias . . . .” (Luke 1:17) Therefore, it must be that certain people who are called of God to be forerunners of the Kingdom, as John was, act in the “spirit and power of Elias.”
This principle must apply to other prophets as well as John, for Jesus not only said that “Elias has come already,” but also that “Elias truly shall first come, and restore all things.” Thus, a restoration would still be needed in the future, just as Latter-day Saints have proclaimed. Noting the many persons the revelations of Joseph Smith identify as “Elias,” modern Apostle Bruce R. McConkie summarized the LDS doctrine: "Elias is a composite personage. The expression must be understood to be a name and a title for those whose mission it was to commit keys and powers to men in this final dispensation."124 And as with the Lord’s first advent, Latter-day Saints believe that Elijah himself was one of the heavenly visitors who appeared to Joseph Smith (as well as Oliver Cowdery) to restore the original “keys and powers.” (See D&C 110)
Although one might argue that Joseph Smith could have extracted this doctrine from the Bible, but the fact that no other group has developed a similar dogma would seem not to support such a conclusion. Here again, Joseph struck upon a prominent doctrine of the early Church that had been lost. For example, Hippolytus (ca. 200 A.D.) indicated that various forerunners would appear to prepare the way for the second advent of the Savior:
[The Savior] is to be manifested again at the end of the world as Judge. It is a matter of course that His forerunners must appear first, as He says by Malachi and the angel, [Malachi 4:5-6]. These, then, shall come and proclaim the manifestation of Christ that is to be from heaven; and they shall also perform signs and wonders, in order that men may be put to shame and turned to repentance for their surpassing wickedness and impiety.
Justin Martyr explained the doctrine of Elias in similar terms to Trypho the Jew, asking, “shall we not suppose that the word of God has proclaimed that Elijah shall be the precursor of the great and terrible day, that is, of His second advent? . . .” 126 According to John Chrysostom, John was to be the forerunner of Christ’s First Advent, and Elias would be the forerunner of the Second: "John is Elias, and Elias John. For both of them received one ministry, and both of them became forerunners."127 Similarly, Victorinus, Methodius, Cyprian, Lactantius, Jerome, Augustine, and Theophylact all expressed the belief that Elijah would come to “restore all things” before the Second Coming of the Lord.
Therefore, it is safe to say that the early Christians had a certain belief that the way would be paved for the Lord’s Second Advent through the agency of various prophetic forerunners, including Elijah the prophet, who the Savior said would come and “restore all things.” And it is also safe to say that Joseph Smith preached a very similar doctrine. What is more important, this belief of the early Church, combined with the evidence for the apostasy, underscores the fact that a restoration of the gospel was necessary and expected.
fairlds.org/Restoring_the_Ancient_Church/chap02.html