In this, the dispensation of the fullness of times when there is to be a "restitution of all things" (Acts 3:21), perhaps no Old Testament prophet has a more significant mission than Elijah, the last prophet in the Old Testament to hold the keys of the priesthood.
. . . Without the keys which Elijah conferred upon the Prophet Joseph Smith in the Kirtland Temple, no ordinance of the gospel would be binding beyond this life. All sealings would be null and void, including eternal marriages and eternal families. Since eternal marriage is a prerequisite for exaltation, our purpose for coming to this earth would be thwarted. The Lord has stated that marriage and families are necessary in order for the earth to "answer the end of its creation" (D&C 49:15-17). Thus, without the sealing powers restored by Elijah, truly "the whole earth would be utterly wasted" at the Lord's coming (D&C 2:3).. . . Although the wording of Malachi's prophecy concerning Elijah's return, as recorded in scripture, varies somewhat, all speak of fathers, children and hearts of children turning to their fathers. In addition, Moroni's words to Joseph Smith refer to promises made to the fathers by the children.
Who are the fathers and children referred to? President Joseph Fielding Smith said:
"The fathers are our dead ancestors who died without the privilege of receiving the gospel, but who received the promise that the time would come when that privilege would be granted them. The children are those now living who are preparing genealogical data and who are performing the vicarious ordinances in the temples." (Doctrines of Salvation 2:127.)
What are the promises? The prophets have taught that we made sacred covenants prior to coming to this earth. President Spencer W. Kimball said:
"We made vows, solemn vows, in the heavens before we came to this mortal life. . . . We have made covenants. We made them before we accepted our position here on earth. . . . We committed ourselves to our Heavenly Father that if He would send us to the earth and give us bodies and give to us the priceless opportunities that earth life afforded, we would keep our lives clean and would marry in the holy temple and would rear a family and teach them righteousness. This was a solemn oath, a solemn promise." (Spencer W. Kimball, devotional address given at the Institute of Religion, University of Utah, Jan. 10, 1975.)