Joseph Smith gave to us a remarkable account of his vision of the eternal worlds that we scarcely understand. He explained that if God willed it, and if the Saints were prepared for it, he could have revealed a hundred times more about the eternal kingdoms than he did in what is now D&C 76 (see HC 5:402).
Juxtaposed with the view of eternal glory is a view of eternal damnation. Joseph saw both. You don't encounter the one without encountering the other. You do not encounter God without also encountering Satan (Moses 1:9-12; JSH 1:15-17).
After testifying that he and Sidney saw the "glory of the Son, on the right hand of the Father," he explained that he also witnessed Satan:
"And this we saw also, and bear record, that an angel of God who was in authority in the presence of God, who rebelled against the Only Begotten Son whom the Father loved and who was in the bosom of the Father, was thrust down from the presence of God and the Son,
"And was called Perdition, for the heavens wept over him - he was Lucifer, a son of the morning."
"And we beheld, and lo, he is fallen! is fallen, even a son of the morning!
"And while we were yet in the Spirit, the Lord commanded us that we should write the vision; for we beheld Satan, that old serpent, even the devil, who rebelled against God, and sought to take the kingdom of our God and his Christ -
"Wherefore, he maketh war with the saints of God, and encompasseth them round about" (D&C 76:25-29).
Joseph Smith learned more about this than we are comfortable considering.
Satan was an angel of God. This is one of two types of beings in heaven (D&C 129:1). He was in authority in the presence of God. What does that mean? Is this the same kind of authority the great and noble ones had "in the presence of God" (Abr. 3:22-28)? What kind of authority must one have to organize a kingdom in heaven in opposition to the Kingdom of the very God of heaven? "[T]he Devil Lusifer also organized his kingdom in opposition to overthrow gods kingdom & he became the son of perdition" (WJS, p. 8). How could he have supposed he could "take" the kingdom of our God and his Christ?
How is it that "many followed after him," and "the heavens wept over him" (ibid.)? What kind of authority and persuasion must one possess to convince a third part of the hosts of heaven that God's plan was not perfectly just and merciful (D&C 29:36)? That they should not trust in Christ, the eternal God?
What does it mean to be a son of the morning? The morning of what?
What does it mean that he fell? Fell from what?