Disclosure: I was provided with a pdf copy by James Pozenel Jr., without which this listing would not exist. Thank you, James!
There are a lot of these small-run items put together, which get played at one or more conventions, and then become extremely hard to find. Most of them push the bounds of the game in some extreme direction, and offer a fun diversion, but then become difficult to work into a long-term campaign.
Demon Cult Classics does not follow that model, but rather offers an elegant system for handling demonic cults that should be usable in almost any campaign.
At its core, Demon Cult Classics uses the four-PC funnel character sheet as a way to model a sect of a demonic cult. Instead of Luck, the sect has Favor. If your Favor falls below your Personality, you receive disapproval. If your Favor ever reaches 3, you are dragged down to hell. Simple, elegant, and easy to understand. There is more to it than that, of course, but the system is clear, and is neither so under- or over-powered as not to be usable with levelled characters as well as with Zeroes. You could have a party of PCs, each run by a different player, who were part of the same sect, and this system would work.
Specific rules for Cultists of Astaroth and Cultists of Paimon are provided. The aspiring judge could easily create more. Likewise, the aspiring judge could create patron write-ups for Astaroth and Paimon (or other demons of their choice), which would help when using this material with higher-level characters.
The author makes a great point of saying
THIS SHIT IS NOT REAL.
If there's anything about this that confuses you, if you're unclear about any of this, if you disagree with me on this point, anything you find herein in no way justifies, validates or in any way provides you with anything you would need in order to do anything illegal, immoral, or otherwise.
For the rest of us, what an amazing product! I would gladly shell out cash for an expanded version of this. Sadly, I know of no place where it is currently available.