4 ¶ Remember ye the law of Moses my servant, which I commanded unto him in Horeb for all Israel, [with] the statutes and judgments.
5 Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:
6 And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers, lest I come and smite the earth with a curse.
We learn from the Prophet Joseph Smith that the law revealed to Moses on Horeb (Sinai) was never revealed to the children of Israel as a nation.
How shall God come to the rescue of this generation? He will send Elijah the prophet. The law revealed to Moses in Horeb never was revealed to the children of Israel as a nation. Elijah shall reveal the covenants to seal the hearts of the fathers to the children, and the children to the fathers.
The anointing and sealing is to be callled, elected and made sure.
(Joseph Smith, Teachings of the Prophet Joseph Smith, selected and arranged by Joseph Fielding Smith, p.323)
As Latter-day Saints we are aware that Moroni quoted part of these verses differently,
39 He also quoted the next verse differently: 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 fathers. If it were not so, the whole earth would be utterly wasted at his coming.
Again, President Joseph F. Smith referred to these verses in his great vision of the spirit world,
47 The Prophet Elijah was to plant in the hearts of the children the promises made to their fathers,
48 Foreshadowing the great work to be done in the temples of the Lord in the dispensation of the fulness of times, for the redemption of the dead, and the sealing of the children to their parents, lest the whole earth be smitten with a curse and utterly wasted at his coming.
The Prophet Joseph Smith also referenced these verses in telling of the visit of Elijah when he brought the sealing powers,
13 ...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--
There are several ramifications to the promises made to the fathers that are to be planted in the hearts of the children. The fathers, in this instance, are Abraham, Isaac, and Jacob. Part of the promise is that they and their posterity are to inherit the earth as a Celestial Kingdom, all being sealed together in the New and Everlasting Covenant (D&C 88:17-20). The other part of the promise is that each one of them are sealed as husband and wife and are priests and kings, priestesses and queens modeled after their heavenly parents (D&C 132:29-33). This is all part of the Abrahamic Covenant that is designed to exalt our Heavenly Father's children.
If there is no one sealed together so that they can inherit the earth as it becomes the Celestial Kingdom, we can certainly see how it would be wasted, as it would sit empty without any inhabitants.
Here we see just the opposite of what the Malachi said of the wicked in Malachi 4:1, that they would be left without root or branch. The righteous have the promise of the coming of Elijah to seal them to their progenitors and their posterity and to their spouses, thus blessing them with both roots and branches. Therefore, the holy Tree of Life becomes alive and eternal with its roots and branches fastened in place.
Several years ago, I gave a talk at BYU Women's Conference on the Abrahamic Covenant. If you are interested you can email me and I will attach a copy of the talk. firstname.lastname@example.org