Heresy is any belief, act, or association judged to betray the sanctioned faith and authority that bind humanity together. In a civilisation that treats loyalty and orthodoxy as matters of survival, the label covers an enormous range of offences, from open worship of dark powers to subtler crimes of doubt, forbidden knowledge, or contact with the alien.
Because the accepted creed centres on reverence for the Emperor of Mankind and unquestioning obedience to established order, heresy is regarded not merely as error but as spiritual contamination, a rot that can spread from a single mind to consume worlds. Its causes are many: the seductive whispers of the Warp, the corruption of ambition, the despair born of endless hardship, or simply the asking of questions deemed too dangerous to voice.
The response is correspondingly severe. Specialised agencies exist to root out deviance, and suspected heretics may face interrogation, purging, or execution, often alongside everyone thought to have been tainted by association. The word carries such weight that it can justify almost any measure taken in the name of purity, up to and including the destruction of entire populations. To be branded a heretic is thus one of the gravest condemnations the setting offers, marking the accused as an enemy not only of the state but of the very idea of order the state claims to defend.