Remarkably, the Savior spends a considerable amount of time discussing prophecies related to the latter-days. Apparently, this was done more for our benefit than for the Nephites (3 Nephi 26:2,6-8). But the scriptural legacy must have been important for the Nephites; Moroni was certainly familiar with this scripture and quoted it, along with portions of Malachi 3, to Joseph Smith when he first appeared to him (JS-Hist 1:36-39). This underscores the importance of Joseph Smith’s mission to restore those priesthood keys and ordinances designed to save all mankind from destruction. It also suggests that even in Joseph Smith’s day, the coming of the Lord was not far distant—speaking after the manner of the Lord (DC 64:24).
The Dayshall Burn Them Upthat It Shall Leave Them Neither Root nor Branch
"The 4th chapter of Malachi is about the second coming of the Lord, as we know. We also know plainly that verses 5 and 6 speak about Elijah’s return and has reference to the life saving ordinances we perform in the temples; namely, baptisms and sealings for our kindred dead. Verse 1 makes us wonder, what is meant of the statement that the burning day that cometh ’shall leave them (the wicked) neither root nor branch’. This too is a direct link to the life saving work that takes place in the temples.
“A plant, is derived from its root; much like we are brought forth from our ancestors. A plant puts forth offshoots, or branches; much like we produce offspring. The great work of baptisms and sealings keeps the roots and branches intact as families are sealed together for life and all eternity. Those that are found to be wicked in the ’day of the Lord,’ will be left with neither root (ancestors), nor branch (offspring), because they will be destroyed. Hence, they are left out of the linking process that transpires through the sealing ordinance. I have gained a deep appreciation for the temple work we do and have come to understand that temple work is the most important thing we can do in this world to save, not only our kindred dead, but ourselves.” (John Cyr, personal communication)
Theodore M. Burton
“In order to understand this passage of scripture, for root read ‘progenitors’ or ‘ancestors’ and for branch read ‘posterity’ or ’children.’ Unless, then, through obedience to the laws of God you can qualify yourself to go to the temple and have your family sealed to you, you will live forever separately and singly in an unmarried state. It seems to me that would be a very lonesome type of existence—to live without the warming influence of family life among those you love, who in turn love you.” (Ensign, July 1972, “Salvation and Exaltation”)