D&C 2:1-2

I will reveal unto you the Priesthood, by the hand of Elijah the prophet, before the coming of the great and dreadful day of the Lord. And he shall plant in the hearts of the children the promises made to the fathers, and the hearts of the children shall turn to their father. D&C 2:1-2

When Moroni appeared to Joseph Smith on the evening of 21 September 1823, he quoted many Old and New Testament passages of scripture. Some of these he quoted quite differently from the King James text. For example, Moroni’s rendition of Malachi 4:5-6 sheds a brilliant light upon an otherwise little appreciated doctrine: Elijah the prophet would be sent to plant within our hearts the promises made to our ancient fathers, Abraham, Isaac, and Jacob – the promise of the gospel, the priesthood, eternal lives, and a land of inheritance (Abraham 2:8/11, 19).This prophecy was fulfilled on 3 April 1836, when Elijah visited the Kirtland Temple with Moses and Elias to confer the keys of the kingdom upon Joseph Smith and Oliver Cowdery. Because Elijah came, there comes into the hearts of God’s latter-day covenant people a desire to have every blessing promised to the ancients. These blessings come only through the sacred ordinances of the house of the Lord. (Robert L. Millet and Lloyd D. Newell. Draw Near Unto Me: Daily Reflections on the Doctrine and Covenants. 12)