How does one define heresy?

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>>54870358
With this!

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HERESY IS TURNING FROM THE PATH OF THE EMPEROR!

>>54870358
Religious thought contrary to the orthodoxy. Funnily enough heresies arise when they see other people as corrupt and strayed from the path, and want to worship the Right and Proper way the god originally intended.

>>54870478
You mean the path set out by the Lectitio Divinitatus, written by...who exactly?

>>54870358

Heathen: Somebody who has not seen the light.
Heretic: Somebody who *has* seen the light and now does it wrong.

So if you land on some backwater world where they worship the relics of an ancient primarch, they're ignorant heathens who don't know better. You take over the place and teach them about the God-Emperor of Man.

When you return 3000 years later and there are two splinter factions arguing whether "The Emperor Protects" each human severally, or all of humanity jointly, then at least one of those sides is engaging in Heresy and must be purged.