You might want to ask in /r/literature/ or /r/AskLiteraryStudies
Dante was well read in Classic mythology. When writing his Divine Comedy he combined Christian teachings with his love for Classic stories. I can think of two reasons for this:
The Bible tells very little of what hell is actually like. If you wrote down all the verses talking about Hell, you could read them in five minutes. So there was lots of room for a storyteller to imagine / interpret / embellish. Storytellers have always borrowed stories, characters, themes, and structures from previous writers and cultures. The Ancient Romans borrowed and repurposed large swaths of Greek mythology. So it should be no surprise that it would happen in the Middle ages. So why was it possible for Dante to do this? Because:
Although Dante lived in a Christian culture, the map of Europe was no longer united in looking towards a single place for art and culture, like Rome had been for so long. Now Europe was a fractured mess of nation and city-states.
I’m not too solid on the religious side of things, but Dante lived in between the two “Great Schisms” of the Christian church: the East-West Schism of 1054 and the Western Schism, fifty-some years after his death. Let’s just say there were a lot of religious debates and changes happening.
The religious climate that Dante lived in was the middle of Scholasticism, when Aristotle and other Classic philosophers were rediscovered by the likes of Thomas Aquinas, etc. It was the merging / reconciliation of Church dogma with classical philosophy. Dante wasn’t trying to rewrite dogma, he was writing a story. So this time period is exactly when you should expect somebody like Dante to fold Classical stories into Christian mythology.
On a related note, although I haven’t studied it in any detail, I’ve always thought it was interesting how Lucifer became the name for the devil before his fall.
Also, for anyone who hasn’t seen them, I’d strongly recommend looking at the illustrations by Gustave Doré of Dante, Milton, and the Bible.