“Elijah Returns and Commits the Keys of His Dispensation as Promised by Malachi”
Doctrine and Covenants 110:13–16
13 After this vision had closed, another great and glorious vision burst upon us; for Elijah the prophet, who was taken to heaven without tasting death, stood before us, and said:
14 Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come—
15 To turn the hearts of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse—
16 Therefore, the keys of this dispensation are committed into your hands; and by this ye may know that the great and dreadful day of the Lord is near, even at the doors.
The Prophet Joseph Smith said:
“‘And I will send Elijah the Prophet before the great and terrible day of the Lord,’ etc., etc. [see Malachi 4:5]. Why send Elijah? Because he holds the keys of the authority to administer in all the ordinances of the Priesthood; and [unless] the authority is given, the ordinances could not be administered in righteousness.”
(Teachings of Presidents of the Church: Joseph Smith [2007], 310.)