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You can adapt elements of a religion and still destroy it. Just look at what inspiration Mohammed took from Judaism and what Daesh would like to do to today's Jews.
Except there is no religion in Imperial Truth. It's
literally atheism that accepts that magic is a thing because it's a thing in Wh40k and it can be explained scientifically, somehow. The Imperial Creed is what's religion (created by one of biggest heretics around, tho) and it was created way too late to actually incorporate parts from Terran religions. Maybe some warped versions of religions from other planets though.
That's just argument by bizarre definition. Christianity itself is only monotheistic because it insists upon a historically unusual definition of what constitutes divinity. They easily fit the definition of gods as imagined by ancient paganism; if anything they don't qualify by virtue of being too powerful
Kind of. If we define gods as magical beings with powers, then Chaos Gods are gods. If we define them as supernatural (magic in Wh40k is a part of nature, kind of) beings that created humans, are timeless, have unlimited powers, being in control of universal laws of nature, etc, they're not gods.
The only reason gods in Wh40k exist, is because someone belives (actually more like emotions) in them (Emperor is kinda an exception, because even if he's reverred as a god, he's still alive), because that's how Immaterium works. This is why Imperial Truth was created, to stop people from worship so they stop making themselves gods that constrain them. If anything, it's humans (or Eldar fucking themselves one, or whatever) that created Chaos Gods. IIRC, Emperor himself calls Chaos Gods... "emergent warp phenomena" or "alien dimension energy thingies that are so fucked up, they think they are Gods" (I'm didin't quote the second one directly)
Also, you're half-right, the Imperial Truth was... not a lie, but it didin't actually mention Chaos Gods because Emperor didin't trust anyone enough to let them know because he thought this would cause people to worship them. It would work better if he was straightforward about it, as witnessed by Interrex with knew about Chaos Gods, didin't think they were gods and Chaos Gods ultimately had to kill them off physically because they couldn't get any religion hooks or stuff.
eeh it's fucking complicated let me get my stuff straight first I am going to sleep now maybe tommorow