The "Turning of the Hearts to their Fathers"

D&C 2:1 Behold, I will reveal unto you the Priesthood, by the hand of ^aElijah the prophet, before the coming of the great and ^bdreadful day of the Lord.

2 And ^ahe shall plant in the hearts of the children the ^bpromises made to the fathers, and the hearts of the children shall turn to their fathers.

3 If it were not so, the whole ^aearth would be utterly wasted at his coming.

D&C 138:46 And ^aMalachi, the prophet who testified of the coming of ^bElijah—of whom also Moroni spake to the Prophet Joseph Smith, declaring that he should come before the ushering in of the great and dreadful ^cday of the Lord—were also there.

47 The Prophet Elijah was to plant in the ^ahearts of the children the promises made to their fathers,

48 Foreshadowing the great work to be done in the ^atemples of the Lord in the ^bdispensation of the fulness of times, for the redemption of the dead, and the ^csealing of the children to their parents, lest the whole earth be smitten with a curse and utterly wasted at his coming.

This morning I heard the words of this prophecy with new ears.

What are the promises made to the fathers?
Promises are covenants.
What covenant did God make with Abraham, and Lehi, and Nephi, and other prophets?
That their seed would receive the gospel in the latter days and be gathered in to Zion.
When we understand this promise that was made and plant it in our hearts, we will understand the magnitude of the prophesied gathering. This promise needs to be planted in our hearts so we can have faith to endure until they are fulfilled.
Why would the earth be utterly wasted at his coming if this promise were not fulfilled?
Because Satan reigns and is moving with lightspeed to stop the work of God. He's trying to destroy faith in this promise that God will intervene. If God does not gather the children of these prophets as promised, they won't receive the gospel and they won't do the work of redemption to seal their own families or their kindred dead.
If the gathering doesn't take place, Satan would gain the hearts of all these souls and they would be destroyed at His coming.
This quote:

Joseph Smith (President)

And now, my dearly beloved brethren and sisters, let me assure you that these are principles in relation to the dead and the living that cannot be lightly passed over, as pertaining to our salvation. For their [the dead’s] salvation is necessary and essential to our salvation, as Paul says concerning the fathers—that they without us cannot be made perfect—neither can we without our dead be made perfect. (D&C 128:15)

made me think that the reason we without them cannot be saved, is that is the covenants made to THEM that we'd be gathered in that saves us. If it weren't for that covenant we'd all be taken over by Satan and he'd destroy this earth and waste it for it's intended purpose.

This link

has some quotes about this scripture.