Is it common knowledge that starships go through the warp and that daemons live there? Do they know about the Gellar field?

Do an average citizen know about rebellions, heretic/chaos cults and aliens? By that, I don't mean what the various cults do or their agendas but rather that they exist at all - and that Chaos and Chaos gods exist. Or are they just taught that everything non-imperial/non-sanctioned is evil and therefore should be destroyed/reported? How much info trickle back from war zones - can average humans separate an eldar from an ork and judge the threats accordingly or are both just aliens to be feared, loathed and killed?

How much do normal citizens know about the Horus Heresy and its outcome? Is it known at all that several legions turned to Chaos, or are the Adeptus Astartes portrayed as divine, pure, impeccable warrior-angels? Is "fallen Space Marine" even a known concept? Would an imperial citizen be able to identify a Chaos marine as a foe (well, some are pretty obviously monsters, like Death Guard)? I mean, Iron Warrior, Alpha Legion and Thousand Sons tend to have few or no obvious mutations and little Chaos iconography, right? With the virtue of ignorance, would normal people even recognize Chaos symbols or regard them as non-imperial? Death symbols exist in abundance in the Empire as well and strange implants/gear/weapons seem not umcommon.

Do normal people even know that the Emperor was alive once, or is it basically "Your God-Emperor watches over you from His Golden Throne on Holy Terra with his mighty, all-seeing eyes like He ALWAYS has"?

I ask, because I feel I get mixed information about it from the various 40K sources. Everything seems to be shrouded in mystery for the acolytes. Not even Adepts get to know Scholastic Lore: Archaic - and they are the loremasters! Only Imperial psykers and Tech-priests get history lessons, for some reason.

Well, the problem is that there is hardly an "average citizen" in the Imperium as they can come from primitive Feral Worlds over far away Frontier Worlds to crowded anonymous Hive Worlds.

Many average citizens do not even know about star ships or only that they exist, but not how they travel. The more educated citizens probably know about star ships and maybe some even that they travel through the warp (or the sea of souls or however one calls it). A few might even know that heinous creatures dwell there that can be a danger for ships, but maybe this “knowledge” is akin to our world’s “knowledge” centuries ago about hobgoblins and ship’s kobolds. So most is sort of a myth for the average citizen. The term "daemon" if at all is used by the Ecclessiarchy as a boogeyman to frighten citizens into religous obedience (like in the real world middle-ages). Knowledge of Gellar fields is restricted and only Adeptus Mechanicus personnel (about Enginseer and above) and certain echelons of the Adeptus Terra (mostly Navy) know of these kind of things.

The average citizen might have heard about rebellions and might have even heard about heretical cults, especially those living in the lower hive regions and the more outlawed regions of a planet. They most probably heard of xenos as well, maybe only as Imperial propaganda (think Starship Troopers). Most citizens do not know about Chaos Gods, and if only as a myth and/or religious anathema preached by the Ministorum.

The “good news” (i.e. propaganda) will trickle back from warzones, though the average citizen would never know where exactly such a warzone is. The average citizen could not separate an Ork from an Eldar (with the exception of those on War or Frontier Worlds maybe) and only knows that the alien is to be feared and destroyed on sight.

They do not know anything except mythical/biblical stories of the Ministorum about the Horus Heresy (i.e. the almighty Emperor and his Arch Angels besting the vile arch-enemy and his devilish hordes). No normal citizen would know that Astartes Legion fell to Chaos.Chaos iconography and symbols often hurt the eye and most citizens would realize something unnatural/unimperial about them. They could not identify a Chaos marine for what he is, maybe just that he does not look friendly.

Whether people know the Emperor was alive depends on the creed the Ministorum teaches on the respective planet. Most of the Ecclessiarchy probably tell of his deeds as a mighty warlord and (living) Emperor, though there are a few planets where the Emperor is worshipped in more abstract ways.

Generally, I consider all the players to be totally ignorant unless they have the requisite lore skill. So, with common lore (Imperium), they know a bit about the empire in general (including history, religion, travel, organization), that there are threats such as xenos, but would not know much about them really.

The players would not have any real idea about daemons if they do not have an appropriate forbidden lore (warp/daemeonlogy). It would just be a another horrible monster that needs killing, but would have no knowledge about tactics or ways to defeat them.

Using this approach, a player form a hive would know more than someone from a feral world. This, at least, works fine for my campaign.

This was a problem at one time for me. One of my player groups knew virtually nothing of Warhammer 40k lore, so as the group assembled for the first time on Bastion Serpentis (Inquisition Headquarters) they got a simple introduction to heresy:

1. The Inquisition kills and suppresses heretics. If you find a heretic, kill them.
2. Anyone (or anything) that wears or is associated with the 8-pointed star is to be killed immediately. They are heretics. (any pictures of said star were displayed under heavy incense and burned afterward)
3. Anyone that wears or is associated with the Inquisition Rosette is a friend.

This of course led to the players question: "Is there any person that uses both symbols?" They were given a cold glare by their master, and sent off on a mission to Piety. While there, they also found a journal by Inquisitor Felroth Gelt… oh what fun I'm going to play on them…

Like Luthor Harkon said, 'average Imperial citizen' is an incredibly broad term; assuming you mean 'typical urban Imperial World-dwelling hab-prol', here is my take on your questions:

vehzeel said:

I'm pretty sure they know about the Empyrean and its dangers- this knowledge would be beneficial to enforcing the belief that their world's very existance depends on the larger Imperium, and the faith of the Ecclesiarchy. Geller fields, probably not- just that the superior intelligence of humanity and faith in the Emperor make it possible for brave souls to travel through the nightmare realm.

vehzeel said:

Clerics frequently preach against treason and heresy, so I'm sure the typical faithful prol will know of- and fear- the possibility of rebellion. And preachers would constantly warn against the corrupting influence of the Ruinous Powers- but these would never be named. With regard to xenos, it is taught that all of them are pure evil and any contact with them is illegal. An Imperial Guard veteran (there are likely a few in any major Imperial settlement) might be able to make a more accurate threat assessment. Maybe.

vehzeel said:

I think they would only know that traitors turned against the Emperor late in the Great Crusade, and that He made the ultimate sacrifice (hence the death symbols) to drive them into the warp and protect humanity. I'm pretty sure that no common citizen is allowed to know that traitor Space Marines exist.

vehzeel said:

It is taught that the Emperor once ruled over humanity personally, and that he will one day be reborn.

Unlike breez, I assume that Acolytes recieve some specialized training upon their recruitment into the Holy Ordos. There are no absolute rules about what a PC knows about the Inquisition and its enemies, so for simplicity's sake I say that PCs know most of the fluff from the Rulebook.

Hope that helps.

Adeptus-B said:

Actually, I'm pretty sure they all state that he still governs over all of humanity from his Golden Throne. Only a certain Inquisitor Creed states that he will be "reborn," iirc, because only the Inquisition and other powerful individuals/organizations know he is forever comatose. I imagine a common heretic, upon hearing the saying "The Emperor wills you to die!!" actually believes the order came from the Emperor himself, while the Judge (not the common Arbitrator) knows it comes from the High Lords of Terra. Just my 2 cents, and I could be wrong. Obviously this is up to each individual GM's interpretation of the fluff and RAW. Anyhoot, other than that small minute detail, I agree with you Adeptus-B.