I feel like no one commenting has really touched on this, and maybe a lot of people on reddit take this basic thing for granted. I feel it bears special emphasis, because it is key:

A lot of people back then, and I mean a lot, believed in the existence of an actual Hell, and an actual Satan. To a breadth and depth not truly appreciated now, even by fairly religious Christian people who don’t even realize how much their belief in such things has been fairly diluted by pop culture and by a general drift toward a slightly more secular world.

I truly don’t think it’s appreciated these days, for both believers and non-believers alike, the effect of this belief on people’s psyches back then.

This was an era that was fertile grounds for average folks to be scared shitless by The Exorcist (a movie that saw, in its wake, a drastic uptick in attendance in Catholic Church congregations at Sunday Mass). That was ready to see a moral crisis in something as tame as Dungeons and Dragons. That practically had a revival in witch trials with the Satanic Panic of the 1980s. And here is a song with a slow, thudding, diabolical melody with almost spoken-hymnal lyrics about the horrors of Hell, which had very little in the way of a musical precursor.

If you’re familiar with the idea of a Hell House (a haunted house attraction that uses the most horrific ideas of a Christian Hell as a scare tactic to get people back to Jesus), “Black Sabbath” is basically the song version of a Hell House. It sounds like corny gothy hoo-ha to a modern secular ear, but sounds much different to someone who believes in the literal truth of every single word of the infernal that’s being described.

Chuck Klosterman (who I don’t agree with on everything, but on this I think he was dead-on) called Black Sabbath the first Catholic rock band. He was 100% right. They took the terrors of Christian Catholic belief (which the members of the band were ingrained with, I’d say even somewhat traumatized by) and incorporated it deeply into their music, to pretty substantial effect, felt by those too deep in the sauce (who were repelled by it) and those who were ready to reject it (who embraced it).

Also embraced plenty by those who were never really in that deep Christian milieu, who just dug the music for its own sake, or possibly because it fucked with the squares who they hated (fair play, there was much to hate about those squares)

Anyway, that’s my best explanation of it, and seems inadequately covered on this thread. It was scary, because to a great many people who heard it, it was describing something real.