D&C 2

Moroni, with a few variations from Malachi, said to the Prophet Joseph Smith -

"...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.
2 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.
3 If it were not so, the whole earth would be utterly wasted at his coming." (Doctrine and Covenants Section 2:1 - 3)

With little fanfare, Elijah has come as the Prophet later recorded -

"...another great and glorious vision burst upon us; for Elijah the prophet, who was taken to heaven without tasting death, stood before us, and said:
14 Behold, the time has fully come, which was spoken of by the mouth of Malachi—testifying that he [Elijah] should be sent, before the great and dreadful day of the Lord come—
15 To turn the hearts of the fathers to the children, and the children to the fathers, lest the whole earth be smitten with a curse—
16 Therefore, the keys of this dispensation are committed into your hands; and by this ye may know that the great and dreadful day of the Lord is near, even at the doors." (Doctrine and Covenants Section 110:13 - 16)

I note that the keys he committed were for the purpose of turning the hearts of fathers and children. Keys that had sufficient power to bind the hearts of families. President Spencer W. Kimball explained -

"Why should he send Elijah? Because he held the keys of the authority to administer in all the ordinances of the priesthood, and without the authority that is given, the ordinances could not be administered in righteousness.

Salvation could not come to this world without the mediation of Jesus Christ. How shall God come to the rescue of the generations? He will send Elijah the prophet. The law revealed to Moses in Horeb never was revealed to the children of Israel as a nation. Elijah shall reveal the covenants to seal the hearts of the fathers to the children and the children to the fathers. The anointing and sealing is to be called, elected, and the election made sure." (Spencer W. Kimball, Ensign, May 1978, p. 48.)

This power was to bind, link, and seal hearts together. At first to annoint or elect and secondly, conditioned upon faithfulness, permantly seal or in other words, the election is made sure. Is there any question that without this power and condition that everything would be a big waste at His coming.