Where did Christians get the idea that Satan rules hell? The Bible is clear that Satan rules the earth temporarily and will be punished in hell at the end of time.

As with most Christian mythology about hell, the answer is "the Middle Ages." As with most Christian mythology about hell, the answer is also "no, not Dante."

Actually, Dante's Inferno and Milton's Paradise Lost tend to be problems when talking about cultural conceptions of Christianity's hell. Their utter dominance in terms of name recognition erases the wilderness of tangent-filled, overlapping, progressing, regressing, dead-ending, and "alt universe" multi-headed hydra of the evolution of Satan and hell. From the New Testament to Milton, we're talking about a millennium and a half over parts of three continents (or five, but for present purposes, really three). Especially given the scarcity of practical details about hell in the NT, that is a LOT of time and space for some really wild ideas to flourish.

In Western Europe, the source for cultural stereotypes here, ideas about hell and the devil gradually coalesce and streamline through mutually influential and referential sources in the fields of mystical visions, theological treatises, folklore, public theatre, church art, and more. This makes it impossible to point to a single manuscript and say, "A-HA! Finally Satan falls and is crowned Homecoming King!" Instead, I want to look at three texts in particular. Two of them, the Gospel of Nicodemus and the Visio Tgnudali or Vision of Tundale, were probably the most influential visions of hell before the Inferno. The third, from Mechthild von Magdeburg's Flowing Light of the Godhead, is especially useful because of scholarly research showing how Mechthild's visions arise out of a mixture of contemporary text, art, and especially drama--it can be considered a creative consolidation of many strands of thought.

First, the Gospel of Nicodemus isn't actually medieval, and it's not the oldest extended description of hell. It's from the mid-300s, and the Apocalypse of Peter (which can generally be summed up as "Don't have sex, kids") has it beat by a couple hundred years. I'm picking it first because its central infernal drama, the Harrowing of Hell, becomes MASSIVELY popular and important in late medieval religious culture. Second, because it does some things with the idea of Satan that we're not used to today.

The Harrowing of Hell is the name given to when Christ descends into hell after the crucifixion, defeats the devil on his home turf, and liberates the Hebrew Bible patriarchs who have been languishing there because they are righteous, but Christ had not yet redeemed humanity/any humans. So the scenes in the Gospel of Nicodemus are set around the Passion, but occur in hell and are told from the perspective of "Satan" and "Hades"...but it is ambiguous whether Hades here is personified or is hell itself. In Nicodemus's Harrowing, Satan is kind of hell's attack-dog: he's sent forth to fight Christ; in some versions of the story Hades actually locks him out of Hades to fight Christ, because they both know if Christ makes it to Hades he'll free suffering souls and prevent a full infernal reaping of souls still to be born.

So what we see here is "Satan" as a mobile underling or "prince." But "Hades"--unclear if Hades or Hell Itself--is at least the voice of what we today would define as the devil figure. There's nothing explicitly to the effect of Hades ruling hell or a notion of kingship (some translations do call Nicodemus's-Satan "prince"), but Hades is a pretty powerful actor and commander right up to the inevitable point of failure.

Second, I want to jump way ahead to the mid-1100s for the enormously popular Visio Tnugdali, or "Vision of Tundale." This one comes out of a monastery in Germany originally, but ends up in basically whatever European vernacular you could possibly want. Tundale is an exemplary protagonist who tours around the afterlife with an angel guide. He spends A LOT of time getting to witness assorted punishments linked to the type of sins people committed. A lot of versions of the text have the torments described all the way down to what the people are wearing.

An interesting note about the Visio Tnugdali is that one of the mid-level punishments, assigned to clergy who break vows of celibacy, is being chewed up (and, by the way, pooped out) by a devil figure frozen in the middle of a lake...

What I want to highlight here, though, is what the tradition of Satan being chained up at the center of hell is doing at this time. He is absolutely and firmly bound, and isn't happy about it. But. According to Tundale's angel (IIRC it's the angel who says this), this position is temporary. Satan must be bound for now because otherwise he'd get free and wreak utter havoc over Earth and humanity. The poem is clear that this state, his being bound, lasts until Judgment Day. After that, Satan the Prince of Darkness will be free within the confines of hell.

Third, let's turn to the Flowing Light of the Godhead, this wide-ranging book of visions, poems, theology, basically every medieval genre (argues Frank Tobin) written by a semi-nun named Mechthild of Mageburg in the second half of the 13th century. Various chapters/genres in the FLG have been identified as drawing their inspiration from artwork, books like the Gospel of Nicodemus, and especially medieval drama. One of the most important results, especially for present purposes, is that this results in multiple, different but at the same time not paradoxical to her, ideas about hell and the devil.

Mechthild has one particularly extensive vision of hell; Lucifer and Satan likewise figure in other parts of her book. Yes, Lucifer and Satan. Different entities.

First, in Book III's splendidly vivid description of her own visionary visit to hell, Lucifer is indeed bound in the lowest rocky depth of hell. However, he is the undisputed master of the terrain. In contrast to a Dante or an anonymous author of Tundale, Mechthild sees Lucifer administering the punishments to the whole host of types of sinner. Lucifer is bound in terms of straightforward description, but spatially-functionally as the narrative goes, he's not. He gets to eat, fart, shit, wear perfume, argue, move around among groups of sinners and the same time he sits near some of them, &c (basically, what devils on medieval stages get to do).

Lucifer sits bound by his guilt in the deepest abyss...[he] grabs the proud one and thrusts him under his tail and says: "I have not sunk so deep that I shall not lord it over you." All the sodomites pass down his throat and live in his belly. Whenever he draws a breath, they slide into his belly. But when he coughs, they are expelled again.

The false saints he puts upon his lap, kisses them hideously...Unceasingly he gnaws the usurer and rebukes him for having have been moved by mercy. The thief is strung up by the feet to serve in hell as a beacon, but the damned do not see the better for it.

Much later, however, Mechthild has shorter visions in which Satan and Lucifer figure as separate characters. They center around the Nativity rather than the Passion, but there are major traces of the Gospel of Nicodemus in her vignettes. Most importantly, Satan is once again the topside attack-dog. Lucifer is presented much more as a character than Hades...except for one line where Mechthild refers to Lucifer "snapping his hellish mouth." We've learned earlier in the text what a "hellish mouth" really is:

At the top, hell has a head that is hideous, and has on it numerous fierce eyes which shoot forth flames.

In western medieval art, indeed, the Hellmouth is the most common entry to hell, furthermore, it's especially prominent in the iconography of the Harrowing of Hell. So here there are definite traces of Nicodemus.

And Mechthild is even more explicit than the late antique text about Lucifer's power within hell, though evidently not to leave it. There's no talk about binding here. Satan addresses/thinks of Lucifer as "master." Lucifer talks about needing baby Christ to commit a sin and doesn't want him coming into hell, because besides freeing souls, Christ is the person who could (but has not yet) judge Lucifer and thus save the Patriarchs but also prevent future generations from automatically falling into the pit.

I hope these three texts have proven good examples to illustrate the complicated evolution of hell and Satan in ancient and medieval Christianity, and the boundaries of what it can mean to rule if one cannot leave.