Ted Sider apparently has an argument (I reporting second-hand) that there is a continuum in the degree of sinfulness, but there is no continuum in the heaven-hell welfare spectrum, since there is a sharp jump in welfare as one moves from an eternity of suffering to an eternity of joy. Therefore, he concludes, divine judgment cannot be just if the outcomes are heaven or hell.
Now one way to answer this is to say that there really are sharp cutoffs in the moral life, such as that between those in a state of mortal sin and those not in a state of mortal sin. The cutoffs would not be defined by some kind of a moral arithmetic[note 1], but by a qualitative fact about the state of the person's will. Thus, Aquinas defines the state of mortal sin in terms of the lack of charity. Now, charity is a fairly sharply defined state of friendship with God (which state is always the fruit of grace). The mortal sinner lacks charity entirely, though the charity will be restored in repentance and forgiveness. Now, there might be a continuum in the degree of charity, say from zero to a hundred, but the difference between zero charity and even the tiniest bit of charity is deeply significant. Even a tiny bit of charity makes one fit for eternity with God (but the more charity there is, the more blissful that eternity will be). But a complete lack of charity makes one fit for damnation.
Is it plausible that there should be such sharp cutoffs in the moral life? Well, what led me to this reflection was watching the excellent 1953 film Pickup on South Street. The central character, Candy (Jean Peters), is a woman who has lived somewhat on the wrong side of the law, and is now trying to leave that life behind, but has one last task of greyish legality. However, she finds that she is enmeshed in a situation of Soviet espionage. And then it becomes clear that she sees a yawning gulf between mere crime and treason, and she assumes, perhaps wrongly, that other people living on the wrong side of the law see it this way, too. It is one thing, in her mind to be a pickpocket (though she is not one herself), and quite a different to work for the Reds. The film makes it plausible that there is indeed a sharp cutoff between other crimes and treason. It's almost as if treason were an allegory for mortal sin. See the film—it is really good. (If you have Netflix, it's available from their Watch Instantly section—that's how I watched it.)