Lol, but the characters in the stories have to have knowledge to some degree, to make them characterful. But generally even the characters have little knowledge, unless they are main. Of course everybody in the Ravenor trilogy knows some, otherwise Ravenor would have no business dealing with them. Those people are either his contacts or his targets generally.
Given that quite a few of the people Ravenor has dealings with are civilians, including quite a few Bounty Hunters, Rogue Traders, police/arbites officers and low-class crime bosses, including some that he comes across purely by chance, I think this says a little something about how common knowledge of Chaos is.
But the average citizen knows nothing, and the armed forces know little. Hell, haven't marines been mindwiped before? Or am I thinking about something else there...
Actually, the armed forces seem almost universally to have some knowledge of the Ruinous Powers. Every Guardsman I've run across in fluff, ever, seem to be able to identify chaotic powers as belonging to the forces of chaos, while the higher-ups know specific details about the Dark Gods and the Warp (see Caiphas Cain, Gaunt's Ghosts, Ben Counter's novels, etc.), while every single Space Marine has pretty much as much knowledge about chaos as you would ever need to shoot and/or stab it, while Chaplains and Librarians know quite a bit more than that. That's also not even getting close to Inquisition and Ecclesiarchal forces, which would have an education well beyond most Space Marines. Also, Space Marines don't get mind-wiped. If they come into too close of contact with a really, really nasty daemon, they might be executed on suspiscion of contamination, but they're far too valuable and dangerous to be subjected to such a risky maneuver.
So, of course Inquisitors know more than a little.
By extension, so do their retinues.
Space Marines are permitted to know more than most.
Grey Knights of course.
Also SoB
Planetary governors will often know more than they should, but then often draw the unwanted attention of the Inquisition (probably the reason you find that they are in some of the books)
The Authorities of the Imperium try to keep knowledge of the other realm from the general populace to prevent corruption.
I'd agree with you to some extent. The Imperium does
not go out of it's way to give out information about Chaos. They do not want the average joe citizen to know the inner workings of the Warp, and they're well justified in that. That said, there's significant evidence to believe that they also don't go out of their way to hide the most general knowledge of Chaos, either. It doesn't seem far fetched to me at all that the Imperium would do the equivalent of "Look at this picture. This disgusting heretic worshipped the Ruinous Powers, betraying the Emperor and all of Mankind, and he's doomed his soul to damnation. Beware the Ruinous Powers and have faith and the Emperor will protect you.". In comparison, anybody who found out how to summon a Daemon, however accidentally, would be executed, whether he intended to use that knowledge or not. Also, I think that Imperial Guardsmen (and possibly PDF forces, but certainly their high-ranking officers) are given something of a crash-course on what Chaos-worshippers are and how best to kill them. It only makes sense to tell your soldiers a bit about the enemy that they are most likely to be fighting (well, Orks excepted) to give them a better chance in combat, and to resist the lures of Chaos.
Basically, what I'm saying is that we know the Imperium heavily restricts functional knowledge of chaos for anyone who isn't a member of the Inquisition, but the general knowledge that there's something out there trying to eat your soul seems to get spread around an awful lot, and as people get more important in the Military or Governmental arms, they are given more detailed (but certainly not too detailed) knowledge of things. Of course, only the very most trusted and important people in the Imperium know all that there is to know about Chaos, but just about everyone we meet in the fiction knows that Chaos exists.
Edited because the quote was messed up...