the nature of sin

[Belacqua]

Mostly the usual Christian prejudice that everybody who doesn't agree with them suffers for eternity. Some of the "noble pagans" merely got Limbo, but Dante has people who lived and died long before Christ burning in the sixth circle with the heretics.

He also shows at least one person not normally thought to be Christian on his way to heaven. He's clear that God decides who goes, and we don't know. He also points out that some proud Christians will wake up on Judgment Day and find some surprising people ahead of them in line. 

You're correct that as a Medieval Christian he thought that bad people would go to hell. But it isn't merely a question of "who doesn't agree with them," it's a question of who is evil. 

It's also hard to miss how both heaven and hell are populated by people that Dante liked or disliked. Inferno is one long grudge list.

There may be some decisions based on grudges. But it's not true to say that it's "one long grudge list." Because there are many decisions not based on grudges. 

If I updated the Comedy today and put Hitler in hell, that wouldn't be due to a grudge; it would be because I thought Hitler was evil. 

Inferno is devoted to a systematic and exhaustive description of all the tortures of Hell. Paradiso has just 4 canti devoted to describing heaven and does so in more general, nebulous terms. I have yet to hear of a description of heaven, by any author, that was anywhere near so detailed as Dante's description of hell.

I appreciate your walking back your earlier claim here. Before you said, "It's a condemnation of humanity that nobody has every penned a systematic description of heaven," but that's not true. Your current claim, that the description of hell is more detailed, is more accurate. 

But remember that for Dante, heaven, like God, is entirely simple, without divisions, time, or space. There is less to describe than there is in hell. It is also extremely difficult to talk about, because humans are animals who live in division, time, and space, and Dante has to make extensive use of the ineffability topos in order to talk about things that language isn't good at describing. Nonetheless, his description of heaven is systematic and theologically in accord with the main Catholic theologians, despite being perhaps not as long as you'd like. 

Anyway, this is getting away from the point I was making earlier: Dante's conception of sin is not a random collection of prejudices or arbitrary commandments, but a systematic conception of how human behavior diverges from what's in our own best interests.