In the Warhammer 40,000 universe, the act of heresy is a religious opinion, act or promulgation of a doctrine that contradicts the beliefs and the commandments of the Imperial Cult of the God-Emperor of Mankind. The commission of heresy is often not merely the expression of an unconventional religious viewpoint, but the act of worshiping the Ruinous Powers of Chaos or aiding the Forces of Chaos in attempting to subvert the rule of the Imperium of Man. No person is beyond the suspicion of the Inquisition, not even the great Space Marines of the Adeptus Astartes. Acts of heresy are catagorized in legal grades (1st, 2nd, 3rd, etc) which differ based on the severity of the act. Punishments range from imprisonment, assignment to an Imperial Guard Penal Legion, excommunication from the rites and protection of the Ecclesiarchy, execution, and arco-flagellation, to name but a few.

Acts of Heresy include:

• - Worshiping other Gods besides the Emperor, who is the God of Mankind, especially the Chaos Gods. This does not include the Machine God who is worshiped by the Adeptus Mechanicus as a facet of the Emperor.
• - Conducting witchcraft
• - Treason
• - Usurping power. See Horus Heresy and Plague of Unbelief
• - Using alien technology. However, the Adeptus Mechanicus may confiscate alien technology for research purposes)
• - Employing warp-based weaponry and entities, such as in the use of daemonswords and daemonhosts. These heretics are hunted down by the famed inquisitor Covenant.

The responsibility of cleansing the Imperium of heresy belongs to the Ordo Hereticus, the most feared Ordo in the Imperium.

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