D&C 128

After having quoted Malachi in the -

"...last chapter, verses 5th and 6th: Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the Lord: 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."  (Doctrine and Covenants | Section 128:17)

The Prophet explained -

"I might have rendered a plainer translation to this, but it is sufficiently plain to suit my purpose as it stands. It is sufficient to know, in this case, that the earth will be smitten with a curse unless there is a welding link of some kind or other between the fathers and the children, upon some subject or other—and behold what is that subject? It is the baptism for the dead. For we without them cannot be made perfect; neither can they without us be made perfect. Neither can they nor we be made perfect without those who have died in the gospel also; for it is necessary in the ushering in of the dispensation of the fulness of times, which dispensation is now beginning to usher in, that a whole and complete and perfect union, and welding together of dispensations, and keys, and powers, and glories should take place, and be revealed from the days of Adam even to the present time. And not only this, but those things which never have been revealed from the foundation of the world, but have been kept hid from the wise and prudent, shall be revealed unto babes and sucklings in this, the dispensation of the fulness of times."  (Doctrine and Covenants | Section 128:18)

What this suggests to me is that we must be linked or sealed together as a single family of the House of Israel or family of Christ to receive the blessing of exaltation have received all necessary ordiances and covenants.  This is the work of salvation.

And this can only take place unless work or a righteous offering for the dead is performed in Holy Temples -

"For we without them cannot be made perfect; neither can they without us be made perfect. Neither can they nor we be made perfect without those who have died in the gospel also..."  (Doctrine and Covenants | Section 128:18)

Thus the words of John the Baptist become clear concerning those of the Aaronic Priesthood or the Levitical Priesthood give of their time in serving in the temple for -

"...he shall purify the sons of Levi, and purge them as gold and silver, that they may offer unto the Lord an offering in righteousness."  (Doctrine and Covenants | Section 128:24)

I believe that by performing baptisms for the dead, young priesthood holders offer a righteous offer to the Lord.