With gratitude to SepehrNS for the tag! I have a couple of earlier answers that might help. The closest one is:
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Yeah, yeah, Dante always gets all the credit. Don't get me wrong--the breathtaking scope, literary dexterity, theological and political ambition of the Commedia make it a suitable and attention-grabbing reference point. But Dante's versions of hell, purgatory, and heaven are deeply rooted in more than a millennium of Christian visions of the afterlife (influenced by various pre-Christian and philosophical traditions as well). For the early Middle Ages up through 1200, in fact, visions of heaven and hell were the single most popular form of religious vision narrative in the Latin west. (They don't exactly diminish in popularity afterwards, but other forms of visions become cultural tsunamis). Don't worry; for this answer I will concentrate on hell. ;)
The Bible does give us the idea of a "lake of fire," but it's remarkably scant on other details. Nevertheless, already in the New Testament apocrypha (texts written a bit later than the canonical NT books), there are extensive visions of the afterlife in the Apocalypse of Peter and in the Shepherd of Hermas. The Apocalypse of Peter is intriguing because it already develops the idea of the contrapasso, or "the punishment symbolizes the sin."
Early medieval voyages to the afterlife often take on, as a whole, the tone of a morality tale. A newly deceased sinner is given a cosmic tour, only to come back to life to spread the message of the joys of paradise/horrors of hell (and, of course, they inevitably clean up their life). One of the innovations of this era, that Dante will also draw out, is the use of hell visions as political weapons. More often this is much vaguer than Dante, like placing all the bishops who buy and sell offices in hell. But there's a reason for that. These visions are written and passed down as experiences that "actually happened"--as genuine witnesses to the underworld.
By the high Middle Ages, there's a new wrinkle in the medieval visionary tradition: the idea that "living saints," particularly women, are specially graced by God with divine visions. Although much of the women's visionary corpus is Christocentric, many visionaries receive (or ask for and are denied, as with Julian of Norwich in the 14-15C) a short trip through heaven and/or hell. Women's visionaries of purgatory, meanwhile, often involve these living saints actually pulling people out according to God's will and then by their own prayerful and ascetic intercession. Mechthild of Hackeborn, a late 13th century German visionary whose visions of paradise sparkle with gemstones and light, is typically proposed as Dante's "Matilda"--that is, that he was familiar with her and her visionary text, the Liber specialis gratie.
But for my money, the best high medieval vision of hell belongs to Mechthild of Magdeburg, an independent religious women slightly older than her co-named monastic counterpart. Mechthild has extensive and lavishly described visions of paradise and purgatory as well, but her hell is top-notch. You can read a fair portion of Frank Tobin's translation of it on Google Books (scroll down a bit on that first page), and you absolutely should.
I have seen a city / its name is eternal hate
It was built in the deepest abyss / From all kinds of stones of huge capital sins
[...] Lucifer sits bound by his guilt in the deepest abyss. There flows unceasingly out of his fiery heart and out of his mouth all the sins, torments, sickness, and shame in which hell, purgatory, and the earth are so wretchedly entangled. In the bottommost part of hell, the fire, gloom, stench, shuddering, and all kinds of intense pain are the greatest.
[...][Lucifer] 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 fo rhaving 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.
[...] At the top, hell has a head that is hideous and has on it numerous fierce eyes which shoot forth flames.
Oh, Mechthild!
I introduce that last bit because there is a parallel tradition that passes down ideas of (especially) hell in the Middle Ages: art. Indeed, depictions of hell were a standard feature in church sculpture, generally as part of a Last Judgment scene. And while Dante's "gates of hell" have their place in iconography (though note that Satan's jaws have a starring role in the lowest level of hell), Mechthild's reference to hell having a head arrives at the iconographic symbol of hell up to the Reformation: the hellmouth. That's right. Long before it was terrorizing the students of Sunnydale High School, hellmouths were all over medieval manuscripts, churches, and imaginations. Sometimes they are more reptilian, sometimes more lion-like, sometimes just abstract evil with eyes.
As far as conceptions of the afterlife go, Dante's importance is less in creativity and more in sealing the deal. The insane popularity of the Commedia and its enduring hold on western imaginations means we are likely to continue to talk of hell in terms of his metaphors and descriptions. But those literary devices have deep and fertile roots.
Greetings. Not to discourage further responses, but do check this older answer while you wait :
Hope this helps.
Dante does have a major influence on the depiction of Hell, but his view of the underworld isn't just about fire. In fact, the very bottom of it (where the worst people are) is extremely cold. This is a good reminder that Italian inferno just means 'below' (from Latin infra).
One of the most influential depictions of the underworld is found in the first-century BCE Aeneid of Vergil—whom Dante chose as his guide for the first 2/3 of the Divine Comedy in part for that reason. In Book 6 of that poem, the hero Aeneas goes to the underworld and sees people punished for their wrongs, grouped into certain places by type of wrong, and the good people experiencing a great afterlife.
Vergil inherited some of this from previous authors (the punishments of Sisyphos, Tantalos, etc. go back at least as far as Book 11 of Homer's Odyssey), but he seems to have innovated a lot. Even after the introduction of Christianity, Vergil was one of the main authors people would study in Latin, so his influence is immense. And it wasn't a big jump to adapt his view of an underworld with people justly compartmentalized to a Christian framework.
I can't speak particularly to the concept of Hell itself but it's worth noting that the Inferno is heavily influenced by medieval literature on Purgatory, specifically a traditional pilgrimage in Ireland. St Patrick's Purgatory is an active site of catholic piligrimage despite several attempts in the seventeenth century by protestant authorities to permanently shut it down. In any case, the traditional account is that St Patrick wanted to demonstrate the existence of Purgatory to the Irish, and so opened a hole in the earth so that it would be possible to travel through Purgatory. The site is an island in Lough Derg, a site with earlier pagan associations (though there is some speculation that the current site is on the wrong island).
The best known account would be the twelfth-century (or possibly early thirteenth-century) Tractatus de Purgatorio Sancti Patricii which narrates the journey of an Irish 'knight' Owen through the Purgatory, as retold by Owen to an English monk, Gilbert, who is the source for the narrator. This chain of separation would later be used to cast doubt on the narrative, with even the very identity of the narrator 'H.', 'a monk of Saltry', sometimes given as 'Hugh', other times disputed as 'Henry of Salisbury'.
According to the Tractatus, Owen first arrives on the island, hoping to atone for his past sins as a knight in the service of King Stephen of England. He spends fiftheen days on the island praying before partaking in a mass. Owen insists upon his intent on entering Purgatory against the advice of the monks there before being locked up in the pit overnight.
In the pit, he encounters a new group of men - all dressed in white - who he assumes are monks, who warn him of the dangers ahead and remind him to use Jesus' name to protect himself. He proceeds deeper into the pit where he encounters many terrible sights. To quote Zaleski's paraphrase:
Owen visits four fields of punishment where, as in earlier visionary and apocalyptic accounts, sinners are devoured by dragons, set upon by serpents and toads, fixed to the ground with red-hot nails, baked in furnaces, immersed to various degrees in boiling cauldrons, and hooked to a flaming version of Ixion's wheel. Moving south, Owen and his demonic guides come upon a well from which naked bodies spew forth like sparks and then fall back into the sulphurous flames. "This is the mouth of hell," the demons say as they cast Owen in. He falls endlessly for this well is none other than the realm of utterly lost souls, the bottomless pit described in Revelation 20:1-3.32. No longer a mere spectator, Owen descends into a deadly state of oblivion, forgetting to call on God's name. Through a last-minute divine intervention, Owen remembers the invocation and a tongue of flame lifts him up to safety.
Owen finds himself crossing a bridge over the flames, too narrow and treacherous to cross easily, but which he finds easier and easier to cross as his journey through Purgatory has cleansed him of sin. He makes his way up to Heaven, stays a while, but is eventually locked out and sent back down. His way down is easy now and the demons retreat from him. He meets up again with the men in white who give him absolution and as he reaches the door of the pit, the dawn arrives and the monks unlock the gate and set him free.
Many of these elements should be familiar to readers of the Inferno, but - as Zaleski notes - the Tractatus is referencing apocalyptic passages from the Bible and early Christian literature, particularly Revelation. It's also important to note that here fire and flame originates in a purgatorial, cleansing meaning - not just for the sake of punishment but as a penance (see LeGoff for more on this).
The Tractatus became popular in continental Europe, though the story of Owen is not mentioned by the contemporary accounts of Jocelyn of Furness and Girlaldus Cambrensis who both describe the pilgrimage. Dante's Inferno is heavily inspired by Owen's story in the Tractatus, as many readers would have likely been aware. Lough Derg became a popular and well-known destination for late medieval and early modern European pilgrims. When Archduke Ferdinand (the future Emperor Ferdinand I) and his entourage was diverted to Kinsale, one secretary, Laurent Vital, inquired at great length about St Patrick's Purgatory (more than 350km away) and was disappointed to learn that the only person in Kinsale to have gone there - a woman - had not had any visions of Hell.
To go into more detail about the piligrimage: with the Reformation, the pilgrimage became hotly contested. To Irish Catholics the piligrimage was obviously quite important, both an affirmation of their faith but also a famous part of the hagiography of St Patrick, whose cult had begun acquiring a strong 'national' element. To Irish Protestants, however, keen to claim continuity with St Patrick themselves, it was important to discredit the piligrimage as fraudulent and unrelated to the saint. One claim was that, rather than St Patrick, the piligrimage had been instituted by Patrick of Dublin, an eleventh-century bishop and so could be dismissed as 'Romish' superstition.
As the Irish protestant bishop and theologian James Ussher put it:
I passe by, that Nennius, and Probus, and all the elder writers of the life of S. Patrick that I have met withall, speake not one word of any such place; and that Henrie the monke of Saltrey, in the daies of King Stephen, is the first in whom I could ever finde any mention thereof.
In 1636, the Lords Justice of Ireland ordered the site be demolished. James Spottiswoode, protestant bishop of Clogher, supervised the destruction and sarcastically wrote back to Archbishop Ussher:
The country people expected that St Patrick would have wrought some miracles; but thanks be to God none of my company received any other harm than the bad waves.
To Irish Protestants like the historian and judge Richard Cox, there was nothing spiritual at all about the cave, much less any portal to Hell:
this Purgatory was found to be a small Cave under Ground, where the Damps arising from the Earth, so influenced crazy Melancholy People, as to make them dream or fancy whatever they were beforehand told they should see.
However, within a decade of the destruction Jesuits were able to restart and partially rebuild the piligrimage and - despite recurrent efforts in the remainder of the seventeenth century to shut it down - pilgrimage continues to today.
Sources:
Jacques Le Goff, The Birth of Purgatory trans. Arthur Goldhammer (1984)
Bernadette Cunningham and Ray Gillespie, 'The Lough Derg Pilgrimage in the Age of the Counter-Reformation', Éire-Ireland, 39:3&4 (2004), pp 167-179.
Carol Zaleski, 'St. Patrick's Purgatory: Pilgrimage Motifs in a Medieval Otherworld Vision', History of Ideas, 46:4 (1985), pp 467-485 (Zaleski gives the best summary of the Tractatus I have to hand, but I have read another elsewhere which I cannot locate right now!)
Fiona Rose McNally, The Evolution of Pilgrimage Practice in Early Modern Ireland (MLitt Thesis, NUIM, 2012): online.
James Ussher, A Discourse of the Religion Anciently Professed by the Irish and Brittish (1631).
Henry Jones, Saint Patricks Purgatory... (1647).
John Lynch, Cambrensis Eversus (1662) (Lynch's book is a catholic critique of Giraldus Cambrensis - and successive English writers on Ireland - and contains a short defence of the account given in the Tractatus)
Richard Cox, Hibernia Anglicana (2 vols, 1689-90).
(On Ferdinand and Vital's time in Kinsale, Hiram Morgan, 'Sunday 6 June 1518—the day the Renaissance came to Ireland', History Ireland, 20:3 (May/June 2012))
I want to chime in with a different corner of medieval Europe - in the North (both England and Scandinavia) in the early Middle Ages, there's a portrayal of Hell as a place of extremes, both hot and cold, which is familiar from Inferno's bottom layers. But the depiction is slightly different - the damned would stand in blazing fires, then be forced to leap into the frigid snowdrifts when the fire grew too much, and by crossing the border, both torments are enhanced. Paul Langeslag explores this idea farther, and links it a conceptual year of only 2 seasons in early medieval England and Scandinavia. Unfortunately, due to the pandemic, I'm not able to access the book to go into more detail, but it's a very strikingly different understanding of Hell than modern ideas of perpetual heat. (In that understanding, Paradise was perpetually temperate, which compared to Iceland sounds pretty good)
However, I don't want to push too hard - the image given from saga evidence about Hell is not consistent in this realm of extremes. Part of Olafs saga Tryggvasonar, recorded in the 1380s but probably composed in the 1100s, is Þorsteins þáttr skelks, in which an intrepid Icelander goes to the toilet alone against King Olaf's orders and runs into a demon. They fall to talking and Þorsteinn asks the demon what Hell is like, and who endures it best and worst. The demon answers that Sigurðr Fáfnisbani endures the torment the best, and that his torment is to kindle an oven (by being the kindling). Meanwhile, Sigurðr's almost-as-famous rival, Starkaðr, endures Hell the worst, being submerged up to his ankles in flames (he's submerged head-first). This gels fairly well with modern ideas of hell as fire-place! Luckily, though, before Þorsteinn gets first-hand experience of what Hell is like, he tricks the demon into screaming loud enough (in emulation of Starkaðr) that he wakes up King Olaf, who rings the church bells and scares off the demon.
Yet a third image of Hell comes from one of the oldest Norse sagas - Niðrstigningar saga, which is a translation of the Harrowing of Hell from the apocryphal Gospel of Nicodemus. In it, descriptions of Hell are sparse, except that it is very dark, has a palace, and gates of brass. The saints are able to talk amongst themselves, though they are chained with fetters, and it otherwise has no particular references to Hell as a place of torment.
This is by no means exhaustive, even among the Icelandic corpus, to be clear - the Hellmouth mentioned by u/sunagainstgold is also clearly attested in medieval Iceland (and in fact, the mountain Hekla was sometimes understood to be the actual entrance to Hell) but hopefully it gives some sense of just how wide the diversity of understandings of Hell were in just medieval Christian Iceland!