Dante's Inferno was considered the result of a divinely inspired dream on the part of Dante. The idea being that god gave Dante a dream of all the things that happened in the inferno, which Dante then recorded in the form of an epic poem.
Oftentimes when someone had what they claimed to be a divinely inspired vision or dream, the nature of the dream was evaluated by a panel of priests who would judge the dream for how well it aligned with biblical theology. If it matched up, your vision was real. If there were discrepancies, you were a liar, or a victim of the devil, and would be punished. Biblical Vision Literature was considered 100% non-fiction for the time, angels were considered to be 100% real, and they would 100% give people divine visions.