I think Dante probably learned about the depths of hell and the consequences of sin. I suppose that’s a pretty obvious answer, but that’s what I see in the poem. When Dante and Virgil are climbing out and back towards earth the poem says, “We never thought of resting while we climbed”. So after nine circles of some serious trauma exposure, as exhausting as that must have been, the journeying men are booking it out of there. Dante the character was exposed to evidence that there is corresponding punishment in hell to one’s sin on Earth. More broadly, there is corresponding consequence to our actions in life. Dante learned that this is not the sort of thing to take lightly, either because it will result in damnation or because our actions and behaviors in life matter and have effects.
What I learned from this poem is that through the lens of Dante’s day and time, the view of sin is very absolute and lacks the perspective that I think is necessary to really judge what consequences of actions may or may not be. We all know that decisions are complex, that most situations lie in a shade of grey. That is not to say that right and wrong don’t exist to some extent or that there aren’t real consequences to what is referred to as sin in this context, but I think extremism can be really dangerous and I see this story as sort of extremist viewpoint in terms of sin and sinners. For instance, in no worldview of mine is there the possibility that “sodomites” are swimming in any part of hell for homosexuality.