CLICK HERE to read 3 Nephi 25.
Compare to Malachi 4.
This shall be the burning, says the Savior (verse 1). We must be burned, for it is the only thing that purifies us and changes our very nature. We can choose to be burned now, by the promptings and calls to repentance that the Holy Spirit gives us as we pray and read our scriptures and listen to the prophets; or, we can be burned in the last day (verse 3). If we choose to be burned now, by the Holy Spirit, and so exchange what is not-of-God for His-righteousness, then in the last day we will be prepared and protected and cared for in nurturing ways (verse 2).
This has been the law, always, even as recorded by Moses (verse 4).
Elijah will come before the last day, He says (verse 5).
We know that Elijah has since come already (see the Topical Guide).
These are our two witnesses for the gathering of His people: Moses and Elijah.
Moses restored the keys for gathering the living saints, and Elijah restored the keys for gathering those on the other side of the veil.
On both sides of the veil, the gathering is done through the Temple ordinances.
The blessings of participating in this work include both the physical (temporal) and spiritual blessings of our families, as helping in the work gathers us further (both physically and spiritually).
And he shall turn the heart of the fathers to the children,
and the heart of the children to their fathers…
This has been the whole purpose of the whole plan, since the beginning (verse 6).