Heresy

Heresy is the theological or religious opinion or doctrine against or held in contradiction to the doctrines and beliefs of the Imperium. By extension, heresy is an opinion or doctrine in philosophy, politics, science, art, etc., at variance with those generally accepted as authoritative.

This means that anyone who is against the Emperor or rules and doctrines of the Empire is guilty of heresy. Such heretics are considered "the enemy within" and fall under the juridiction of the Inquisition.