system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] How much do the citizens of the Imperium know of the Horus Heresy?

In several of his memoirs Commissar Cain references Horus specifically, however I have yet to see another reference throughout the collective writings of the Black Library or anything else.

As an Emperor fearing citizen obviously studying the Heresy would be, well, heresy, but at what level would the knowledge of the Heresy begin to be taught, and how much would be taught?
gpt
Most Imperial citizens are going to be aware of the Horus Heresy, because it was the pivotal element in the Emperor's ascension to the Golden Throne, but the details, particularly pertaining to Horus' allegiances, would be almost entirely unknown.

To your average bloke, it's a God-Lucifer arangement. The Emperor created the Primarchs and the Legions, and Horus got too big for his britches and rebelled against his rightful master. The involvement of Chaos, and in fact the simple existence of anything living in the Warp, would be almost entirely beyond the knowledge of the average person.