The Three Degrees of Glory is one of those Mormon doctrines that at first glance seems random and maybe kinda trivial.
The most fundamental apologetic for the multiple kingdoms of the afterlife is that it fits with our experience. Some people are saintly, some people are quite wicked, and most people are decent but flawed and not striving real hard to get better. That it fits with experience is no small thing. Reality is revelation.
Bruce Charlton offers his own apologetic for the three kingdoms. It’s quite good. It sounds like what he’s saying is that a choice made under duress is not a choice at all, so if God wants people to freely choose Him, the alternative cannot be endless deprivation and total separation from goodness. Put that way, the argument is flawed. A choice made under duress is still a choice and still a valid exercise of agency–you are still free to choose misery or happiness.
But I think what he’s really saying is that one cannot completely choose the relationship with God above everything else unless you have the option of choosing between God and everything else. So ‘heaven’ must include some quite nice options short of complete communion with God. This way we know (and God knows?) that we prefer the glory of His throne above all other things, not just above the fires of hell.
Here’s my own apologetic for the three kingdoms of heaven, written in a review of the Great Divorce. If our only choice is to say yes or no to God, then the complexity of earth life and the many different kinds of good that God offers us would be pointless and unnecessary:
Lewis assumes that any final rejection of God in any sphere of life is tantamount to total rejection. Yet Joseph Smith in vision saw that very few would totally reject God and be cast out to dwell with Satan and his angels. In this vision of the three kingdoms of glory in D&C 76, he learned that even murderers and libertines embrace some good and enjoy a (lower) kingdom of glory; likewise with those who are merely not very active in doing good. And the scriptures are full of those who are half way between heaven and hell, like the Jaredite king who ruled his people in righteousness and himself in wickedness, or the rich young man who did good works but couldn,t surrender himself totally into God’s hands by giving up all his wealth. Lewis asserts that the purpose of creation is either to say yes or no to God, and that there is no middle ground. This assertion has support in ancient scripture. [W]o, wo, wo be unto them, saith the Lord God Almighty, for they shall be thrust down to hell! No man can serve two masters. Because thou art lukewarm, I will spew thee out of my mouth. Modern scripture, however, has shown that the actual destiny of souls is more complicated, just as God’s creation of multiple human creatures, endowed with multiple gifts and passions, in a world itself created in many facets, makes for decisions and relations much more complex than Lewis’ simple relation of the soul to God and the single decision yes or no. Lewis is no simpleton, so he recognizes these complexities, but he treats them all as just differing vehicles for saying yes or no to God (though sometimes he seems to be saying that they are merely vehicles, on which more anon). What Lewis never fully addresses is the situation where a soul says yes in one area and no in other. The Great Divorce would hold that an ultimate refusal in one area ultimately eliminates the soul’s ability to say yes in any other, but Lewis provides no argument for why this would necessarily be so.
His portrayal of the extreme psychological tension of saying partly yes and partly no is vivid and convincing, however. It has helped to persuade me that existence in the telestial and terrestrial kingdoms is unnatural enough—as President Monson observed, it’s easier to live the gospel 100% than 98%—that such existence must require some continued infusion of grace to make stable that which would otherwise not be; or else that the coincidence of the final judgment and resurrection occurring at the same time is no coincidence, a determination of the spheres and relations in which a soul will say yes to God naturally accompanying the total remaking of that soul into a being which only acts in those spheres and relations.
Lewis? view that everything reduces to one yes or one no nonetheless makes it hard to explain why God would have created so much complexity in the first place. Contrariwise, recognizing that our God will cherish and preserve even a tiny yes in a minor part of life helps us understand why in His mercy and His desire He has given us so much life with so many minor parts in it.
This is not to say that the LDS cannot find truth and spiritual value in The Great Divorce, any more than they cannot find truth and spiritual worth in the absolutist heaven or hell preaching of ancient scripture. The Great Divorce shows us, as the prophets have warned us, that there is nothing safe or stable in long halting between two opinions. Revealed theology has its own binaries in Spirit Prison and Spirit Paradise, in the divide between the exalted who have completely said yes to God and everyone else (he that is not with me is against me), and in the divide between those who have completely said no to God and everyone else (he that is not against us is on our part). If my tiniest yes matters so much to Almighty God that He will preserve it despite my roaring NO in the rest of my life, as the doctrine of multiple kingdoms of glory implies, then each of my decisions does have the immeasurable significance that Lewis gives it.
I add that if the afterlife includes a timeless component, as Lewis himself implied in his vision of the chessmen, than anyone who has experienced any good at all in this life cannot be wholly damned. Unless they actively decide to reject that past experience, perhaps, in which case their damnation is to be eternally at war with themselves.
I also add that my speculation above that it requires grace for us to be partly good without being wholly good fits with the Mormon notion that each kingdom of glory requires an association with one of the members of the Godhead.