Theists have a wide array of topics on which to disagree. Which god to worship, how to worship it, or -- even within a specific sect of a specific religion -- which acts are "sin" and what steps need to be taken to avoid a bad afterlife outcome. Ah, but that is one broadly shared idea -- that there is indeed such a thing as a "bad afterlife" outcome and a "good afterlife" outcome, a strict dichotomy with a dividing line (somewhere) between those who arrive here and those who arrive there.
The bad afterlife.
As with all other things, the details of this are murky and disputed. Some believe that there is an eternal Heaven or other such situation to which the good folks go, and an eternal Hell or other punishing thing for the rest. Some believe that those who go to Heaven will be literally given better versions of their physical bodies from life on Earth, and will live in a physical Earth-like city with houses and roads (paved in the proverbial gold), and perhaps some other kinds of buildings and amenities, holy tennis courts and sacred golf courses. And it will be an existence of eternal contentment, with no experience of fear or adversity or challenge of any kind (so in those eternal tennis matches, every player wins the match).
And some believe that Hell is equally a place where souls are given physical bodies, only these are for the purpose of receiving endless physical pain inflicted on them, burning fire constantly scorching skin and melting flesh, sharp rocks leaving no spot to lay one's body down, barbed whips wielded by rapacious demonic beings, and other torments imagined by figures ranging from Dante Alighieri to heavy metal music videographers. This condemnation to torture is depicted as endless in time, with punishment continuing long after any wrongdoing on Earth has been equaled.
Others cast Hell as a bodiless place, more a state of being, really, of eternal separation from the god of their religion; and still others contend that there's a Heaven but not actually a Hell, but that bad souls are simply obliterated for all time (immediately, or after some period of punishment), all the information in them lost forever. A few theologies dare to imagine a Purgatory, an in-between place where some souls suffer while awaiting admission to that eternal Heaven, or perhaps while awaiting a determination of where they'll forever go.
The inevitable cutting-off point.
But whatever the design imagined for the afterlife, there is one necessary constant in dichotomous (or even trichotomous) scenarios. Of the billions of people who have lived and died in all of existence, there must be one pair who represent the cutting-off point. There is one person who "gets in" to Heaven despite being the least deserving person of all who do so -- that is, despite being the least deserving person relative to all other people who get into Heaven. And likewise, there must be one person who is condemned to Hell -- whether this means eternal torment, separateness, obliteration, or something yet unimagined -- who is, of all the people slated for that fate the least deserving of it. The person whom, if just one single additional person of all the billions who've ever lived were to be admitted to Heaven, why, he would be the one.
And, given the continuum of behaviours and lives and lifestyles across the estimated 101 billion humans to have been born at some point in recorded history, the last person to "get in" to the good afterlife will be barely, perhaps imperceptibly distinguishable in the balance of good and bad deeds (or thoughts) from the last person locked out of it. Imagining a slightly more (or less) merciful god doesn't do away with this problem. If the next one or ten or thousand people are allowed in, that simply shifts the point of dichotomy down to a different pair of virtually indistinguishable people.
Where this cutting-off point lies is another a mystery. The faithful certainly can't agree on it. Some will assert that all who believe (and only those who believe) in the right faith, and perhaps even in the right sect within the faith, get the good ending -- and that even those who were simply unknowing as to this faith because they were too young or too remote ever to have learned of it, are deprived of the positive afterlife. For these believers, the destiny of the unevangelized, and the incompetently evangelized looms large, because those people are doomed no matter what they did in life. Other believers will insist that faith alone will not suffice, but that even the most faithful must carry out good deeds and avoid bad behavior to qualify. Contrapositively to the first proposition, some even believe that only those who are good people will receive the reward, even if they are of the "wrong" faith. Some sects, like the Jehovah's Witnesses, believe in an absolute numerical cutoff, with only 144,000 getting into Heaven (interestingly, a number representing only a fraction of the number of members of that group now living, meaning even the vast majority of believers will get edged out based on population limits.
Alternatives -- absolute outcomes or infinite options....
There are a few ideas which avoid the jarring absolute of a cutoff point. There are, for example, absolute ideas. Some believe that the good afterlife is a reward which literally nobody merits, making it plausible that literally everybody gets whatever the bad afterlife is. Conversely, some claim that their god's limitless mercy assures that "Hell is empty," and so literally everybody gets the good outcome. Some theologians have even concluded that the only people who actually exist are those who get the good outcome, and all the rest are simply illusions of people.
A number of theological models posit reincarnation, which would allow individuals to simply live life over and over again, until they merit the good afterlife (which is itself an escape from the cycle of reincarnation). At least one commentator has theorized that there is only one soul being reincarnated and we are all incarnations of it, so that we will each of us experience every life which has ever been lived until we ultimately lead the last life leading into the eternal reward. And, naturally, there are other propositions presented in Pantheism and Pandeism, wherein everything was part of our Creator all along anyway, and simply goes back to it (possibly to experience the consequences of their own Earthly actions through the memories of those who experienced those consequences).
And, naturally, whether one sees the dichotomous nature of typical afterlife beliefs as problematic may itself be a matter of perspective. There is a certain amount of black-and-white thinking which permits for a stark split, with trust that the god in question knows just where to make the cutoff, and all above are in fact worthy of eternal Heaven (even if some are just barely worthy) while all below are deserving of whatever Hell befalls them. But, though most every religious person believes that both they and those they care for will end up all together in an eternal Heaven, there must be some doleful tossing and turning in the dark of the night over where, exactly, the cutoff lies, and whether it is imaginable that they'll fall below it -- since perhaps even less than firmly belonging in Hell, nobody wants to be amongst those very last people who failed to make the cut.
