system
Given the following post, respond as if you're commenting on Reddit:
human
[Warhammer 40k]How does the whole "no one but inquisitions and higher-ups know about Chaos" thing actually work? How can it be that all of Cadia has no idea what they prepare to fight for? Or how tons of trained troops are wasted with executions after fighting chaos?

I just don't feel it makes sense on some level. How can the Horus Heresy be such an integral part of the lore without anyone knowing what it was? How do soldiers/space marines prepare to fight and spend their whole lives fighting chaos yet they're killed after seeing them by Grey Knights/whomever?

You can answer in the form of a veteran inquisitioner addressing a new one if that's your thing.
gpt
You can give your soldiers a general idea, but there are documented instances in which **merely speaking the name of a particular daemon has given it enough leeway to force its way out of the warp through the unfortunate sod's corpse into realspace.** Against such a powerful foe, even the merest whiff of knowledge can seem totally unacceptable without the proper defenses. Troops shooting lasguns do not have to understand what they are shooting at. They merely need to shoot.

The loss of a regiment of (for example) highly trained Kasrkin is a small price to pay in the long run compared to the damage that a single possessed soldier could cause, either directly in combat, or subtly by whispering lies into the ears of the powerful. The loss of a hundred regiments of average guardsmen is nothing, not even a blip on the radar. The Imperium can replace them with a single round of draft slips on a single hive world.