2 And he shall plant in the hearts of the children the promises made to the fathers, [ What were the promises made to the fathers? Who where the fathers spoken of here? Abrahamic covenant - Sealing endowment, Eternal increase that can only be had across the altars of the temple. The sanctioning of temple building is essentially synonymous with the sanctioning of kingdom building, of sanctioning the right to create, to create a world or a kingdom of worlds. referring to the Abrahamic covenant and all additional covenants made under that umbrella (e.g., Jacob, Lehi, Nephi, Enos, etc.). These promises are generally the continuance of physical lineage on the earth and in heaven as well (i.e., eternal life/lives per D&C 132, cf. Gen. 12:1-3, Rom. 9:9, Hebr. 11:17, 1 Ne. 22:8-9, 2 Ne. 3:5, 2 Ne. 10:2, 2 Ne. 10:17, 3 Ne. 5:14-26, Morm. 8:21-25, D&C 3:19-20, D&C 107:40, D&C 132:30). In the BofM, another accessory promise is that the record of the descendants of Lehi would go forth to them at a future time to reconcile them to their Lord, cf. Enos 1:15-18. A more spiritual promise is the grafting in of the Gentiles into Israel which commenced with the events of Acts 10, cp. Luke 24:49, Acts 1:4, D&C 88:4, Abra. 2:10-11. Overall, what this simple phrase means is the Lord is setting about the fulfill the Abrahamic covenant and not leave any strings dangling. This two-way promise(sealing) between you and God is bigger than that because additionally it binds us into the family of Abraham and into that Abrahamic Covenant it binds us into our families, it binds us to Christ and it binds us into that whole family. ] and the hearts of the children shall turn to their fathers. [ A couple of things to note in this verse in comparison to Malachi 4. First that it will be done using the priesthood vs 1. Second in this case it is only the hearts of the children to their fathers - performing the vicarious ordinance work, cf.D&C 127, 128 as disbursed to Joseph Smith from Elijah. ]

3  If it [ From verse 1 "I will reveal unto you the Priesthood" which has the power to administer the saving ordinances of the gospel. ] were not so, the whole earth would be utterly wasted at his coming. [ So if we cannot seal children to the parents for eternity, to have family units for eternity the earth would not have fulfilled its purpose, there would have been no reason for it - without the saving ordinances of the Gospel and especially the Sealing of families the earth would have been wasted. "eternal life, made possible by the atonement, is the supreme purpose of Creation. To phrase that statement in its negative form, if families were not sealed in holy temples, the whole earth would be utterly wasted." Russell M. Nelson Conference Report (October1996), 97. ]

[ Excerpt from History of the Church
During the space of time which intervened between the time I had the vision and the year eighteen hundred and twenty-three—having been forbidden to join any of the religious sects of the day, and being of very tender years, and persecuted by those who ought to have been my friends, and to have treated me kindly, and if they supposed me to be deluded to have endeavored in a proper and affectionate manner to have reclaimed me,—I was left to all kinds of temptations; and mingling with all kinds of society, I frequently fell into many foolish errors, and displayed the weakness of youth, and the foibles of human nature; which, I am sorry to say, led me into divers temptations, offensive in the sight of God. In making this confession, no one need suppose me guilty of any great or malignant sins. A disposition to commit such was never in my nature. But I was guilty of levity, and sometimes associated with jovial company, etc., not consistent with that character which ought to be maintained by one who was called of God as I had been. But this will not seem very strange to any one who recollects my youth, and is acquainted with my native cheery temperament.
In consequence of these things, I often felt condemned for my weakness and imperfections; when, on the evening of the above-mentioned twenty-first of September, after I had retired to my bed for the night, I betook myself to prayer and supplication to Almighty God for forgiveness of all my sins and follies, and also for a manifestation to me, that I might know of my state and standing before Him; for I had full confidence in obtaining a divine manifestation, as I previously had done. While I was thus in the act of calling upon God, I discovered a light appearing in my room, which continued to increase until the room was lighter than at noonday, when immediately a personage appeared at my bed side, standing in the air, for his feet did not touch the floor.
He had on a loose robe of most exquisite whiteness. It was a whiteness beyond anything earthly I had ever seen; nor do I believe that any earthly thing could be made to appear so exceedingly white and brilliant. His hands were naked and his arms also, a little above the wrist, so, also were his feet naked, as were his legs, a little above the ankles. His head and neck were also bare. I could discover that he had no other clothing on but this robe, as it was open, so that I could see into his bosom. Not only was his robe exceedingly white, but his whole person was glorious beyond description, and his countenance truly like lightning. The room was exceedingly light, but not so very bright as immediately around his person.
When first I looked upon him, I was afraid; but the fear soon left me. He called me by name, and said unto me that he was a messenger sent from the presence of God to me and that his name was Moroni; that God had a work for me to do; and that my name should be had for good and evil among all nations, kindreds, and tongues, or that it should be both good and evil spoken of among all people. He said there was a book deposited, written upon gold plates, giving an account of the former inhabitants of this continent, and the sources from whence they sprang. He also said that the fullness of the everlasting Gospel was contained in it, as delivered by the Savior to the ancient inhabitants; also that there were two stones in silver bows—and these stones, fastened to a breastplate, constituted what is called the Urim and Thummim—deposited with the plates; and the possession and use of these stones were what constituted "Seers" in ancient or former times; and that God had prepared them for the purpose of translating the book.
After telling me these things, he commenced quoting the prophecies of the Old Testament. He first quoted part of the third chapter of Malachi, and he quoted also the fourth or last chapter of the same prophecy, though with a little variation from the way it reads in our Bibles. Instead of quoting the first verse as it reads in our books, he quoted it thus:
For behold the day cometh that shall burn as an oven, and all the proud, yea, and all that do wickedly shall burn as stubble; for they that come shall burn them, saith the Lord of hosts, that it shall leave them neither root nor branch.
And again, he quoted the fifth verse thus: Behold 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.
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.
In addition to these, he quoted the eleventh chapter of Isaiah, saying that it was about to be fulfilled. He quoted also the third chapter of Acts, twenty-second and twenty-third verses, precisely as they stand in our New Testament. He said that that Prophet was Christ; but the day had not yet come when "they who would not hear his voice should be cut off from among the people," but soon would come. He also quoted the second chapter of Joel, from the twenty eighth verse to the last. He also said that this was not yet fulfilled, but was soon to be. And he further stated that the fullness of the Gentiles was soon to come in. He quoted many other passages of Scripture, and offered many explanations which cannot be mentioned here.
Again, he told me, that when I got those plates of which he had spoken—for the time that they should be obtained was not yet fulfilled—I should not show them to any person; neither the breast plate with the Urim and Thummim; only to those to whom I should be commanded to show them; if I did I should be destroyed. While he was conversing with me about the plates, the vision was opened to my mind that I could see the place where the plates were deposited, and that so clearly and distinctly that I knew the place again when I visited it.
After this communication, I saw the light in the room began to gather immediately around the person of him who had been speaking to me, and it continued to do so, until the room was again left dark, except just around me, when instantly I saw, as it were, a conduit open right up into heaven, and he ascended until he entirely disappeared, and the room was left as it had been before this heavenly light had made its appearance. I lay musing on the singularity of the scene and marveling greatly at what had been told to me by this extraordinary messenger; when, in the midst of my meditation, I suddenly discovered that my room was again beginning to get lighted, and in an instant, as it were, the same heavenly messenger was again by my bedside. He commenced, and again related the very same things which he had done at the first visit, without the least variation; which having done, he informed me of great judgments which were coming upon the earth, with great desolations by famine, sword, and pestilence; and that these grievous judgments would come on the earth in this generation. Having related these things, he again ascended as he had done before.
By this time, so deep were the impressions made on my mind, that sleep had fled from my eyes, and I lay overwhelmed in astonishment at what I had both seen and heard. But what was my surprise when again I beheld the same messenger at my bedside, and heard him rehearse or repeat over again to me the same things as before; and added a caution to me, telling me that Satan would try to tempt me, (in consequence of the indigent circumstances of my father's family,) to get the plates for the purpose of getting rich. This he forbade me, saying that I must have no other object in view in getting the plates but to glorify God, and must not be influenced by any other motive than that of building His kingdom; otherwise I could not get them. After this third visit, he again ascended into heaven as before, and I was again left to ponder on the strangeness of what I had just experienced; when almost immediately after the heavenly messenger had ascended from me the third time, the cock crowed, and I found that day was approaching, so that our interviews must have occupied the whole of that night.
I shortly after arose from my bed, and, as usual, to the necessary labors of the day; but in attempting to work as at other times, I found my strength so exhausted as to render me entirely unable. My father, who was laboring along with me, discovered something to be wrong with me, and told me to go home. I started with the intention of going to the house; but, in attempting to cross the fence out of the field where we were, my strength entirely failed me, and I fell helpless on the ground, and for a time was quite unconscious of anything. The first thing that I can recollect was a voice speaking unto me, calling me by name. I looked up, and beheld the same messenger standing over my head, surrounded by light as before. He then again related unto me all that he had related to me the previous night, and commanded me to go to my father and tell him of the vision and commandments which I had received. I obeyed; I returned to my father in the field, and rehearsed the whole matter to him. He replied to me that it was of God, and told me to go and do as commanded by the messenger. I left the field, and went to the place where the messenger had told me the plates were deposited; and owing to the distinctness of the vision which I had had concerning it, I knew the place the instant that I arrived there. (Joseph Smith, History of the Church Volume 1, Chapter 2, page 10-15)
Excerpt from History of Joseph Smith, by his Mother
From this time until the twenty-first of September 1823, Joseph continued, as usual, to labor with his father and nothing during this interval occurred of very great importance--though he suffered every kind of opposition and persecution from the different orders of religionists.
On the evening of the twenty-first of September, he retired to his bed in quite a serious and contemplative state of mind. He shortly betook himself to prayer and supplication to Almighty God, for a manifestation of his standing before him, and while thus engaged he received the following vision: [Quotation of History of the Church, vol. i, pp. 11-14] When the angel ascended the second time, he left Joseph overwhelmed with astonishment, yet gave him but a short time to contemplate the things which he had told him before he made his reappearance, and rehearsed the same things over, adding a few words of caution and instruction, thus: that he must beware of covetousness, and he must not suppose the record was to be brought forth with the view of getting gain, for this was not the case, but that it was to bring forth light and intelligence, which had for a long time been lost to the world; and that when he went to get the plates, he must be on his guard, or his mind would be filled with darkness. The angel then told him to tell his father all which he had both seen and heard.
The next day, my husband, Alvin, and Joseph, were reaping together in the field, and as they were reaping, Joseph stopped quite suddenly, and seemed to be in a very deep study. Alvin, observing it, hurried him, saying, "We must not slacken our hands or we will not be able to complete our task." Upon this Joseph went to work again, and after laboring a short time, he stopped just as he had done before. This being quite unusual and strange, it attracted the attention of his father, upon which he discovered that Joseph was very pale. My husband, supposing that he was sick, told him to go to the house, and have his mother doctor him. He accordingly ceased his work, and started, but on coming to a beautiful green, under an apple tree, he stopped and lay down, for he was so weak he could proceed no further. He was here but a short time, when the messenger whom he saw the previous night, visited him again, and the first thing he said was, "Why did you not tell your father that which I commanded you to tell him?" Joseph replied, "I was afraid my father would not believe me." The angel rejoined, "He will believe every word you say to him." Joseph then promised the angel that he would do as he had been commanded. Upon this, the messenger departed, and Joseph returned to the field, where he had left my husband and Alvin; but when he got there, his father had just gone to the house, as he was somewhat unwell. Joseph then desired Alvin to go straightway and see his father, and inform him that he had something of great importance to communicate to him, and that he wanted him to come out into the field where they were at work. Alvin did as he was requested, and when my husband got there Joseph related to him all that had passed between him and the angel the previous night and that morning. Having heard this account, his father charged him not to fail in attending strictly to the instruction which he had received from this heavenly messenger. (Lucy Mack Smith, History of Joseph Smith, by his Mother, pages 74-80) ]