The Prophet Joseph gave a sermon just a few months before his martyrdom in which he said:
The Bible says, “I will send you Elijah the Prophet before the coming of the great and dreadful day of the Lord; and he shall turn the hearts of the fathers to the children, and the hearts of the children to the fathers, lest I come and smite the earth with a curse.”
Now, the word turn here should be translated bind, or seal. But what is the object of this important mission? or how is it to be fulfilled? The keys are to be delivered, the spirit of Elijah is to come, the Gospel to be established, the Saints of God gathered, Zion built up, and the Saints to come up as saviors on Mount Zion.
But how are they to become saviors on Mount Zion? By building their temples, erecting their baptismal fonts, and going forth and receiving all the ordinances, baptisms, confirmations, washings, anointings, ordinations and sealing powers upon their heads, in behalf of all their progenitors who are dead, and redeem them that they may come forth in the first resurrection and be exalted to thrones of glory with them; and herein is the chain that binds the hearts of the fathers to the children, and the children to the fathers, which fulfills the mission of Elijah. And I would to God that this temple was now done, that we might go into it, and go to work and improve our time, and make use of the seals while they are on earth. ((History of the Church 6:184))
D&C 2:3 puts a different twist on why this work must be done, “If it were not so, the whole earth would be utterly wasted at his coming.”
One of the great tasks of the kingdom of God on the earth is to perform the sealing and saving ordinances of the gospel for the entire human family. The sealing of the priesthood must become as one unbroken chain from Adam and Eve to the latest generation, and as such will bind the entire human family together. This can only occur in temples of the Lord, which have been dedicated and consecrated for that divine purpose.