Elijah Sent in the Last Days to Restore Priesthood Authority — Robert B. Thompson (October 5, 1840)
We frequently have mention made of the offering of Sacrifice by the servants of the most high in antient days prior to the law of moses, See which ordinances will be continued when the priesthood is restored with all its Authority power and blessings. Elijah was the last Prophet that held the keys of this priesthood, and who will, before the last dispensation, restore the Authority and deliver the keys of this priesthood in order that all the ordinances may be attended to in righteousness. It is true that the Savior had authority and power to bestow this blessing but the sons of Levi were too predjudiced
And I will send Elijah the prophet before the great and terrible day of the Lord &c &c.
Why send Elijah because he holds the keys of the Authority to administer in all the ordinances of the priesthood and without the Authority is given the ordinances could not be administered in righteousness.Instruction on Priesthood, circa 5 October 1840, p. 9, The Joseph Smith Papers.