The formal name for the Imperium is rightly the Holy Terran Empire and like the Holy Roman Empire upon which it is consciously based it is very interested in the religious welfare of its subjects. However, over as vast an area as that controlled by the Imperium and with so many worlds to care for – religious uniformity is impossible.
Because of the diversity of opinions within the Imperial Creed, the line between orthodoxy and heresy is often blurred. Obviously, worshipping any chaos or Xenos deity is forbidden and those who overtly turn from the Imperial Creed are punished by death, as are those who deny the authority of the Adeptus Ministorum. While those who turn to the worship of forbidden gods such as the Ruinous Powers or alien overlords are the most obvious of heretics, many other beliefs have been declared heretical throughout the ages.
What does and does not count as heresy is generally determined by the high officers of the Ecclesiarchy. Massive divisions of the Adeptus Ministorum exist to monitor and study the myriad sects that exist across the Imperium. Highly ranked, free-roaming officers with equal status to a Cardinal seek out and destroy the taint of heresy. These witch-finders often become the scourges of entire sectors. Of course, many of the Imperium’s worlds are scattered and isolated, so a sect may gain prominence on a given world and flourish for many years before a Cardinal or witch-finder arrives from off-world and decries it as heretical. What happens next will depend upon the character of the officer. Some may convene courts of assize, sitting in judgment over those accused of heresy. Others are more wont to summarily execute sect leaders and instigate a worldwide program of religious re-education to purge the taint of errant doctrine. The most extreme, such as the notorious Witch-finder Tannenburg, put entire worlds to the pyre on the merest suspicion of heresy.
The accusation of heresy is often used as a weapon by those wishing to gain power over others. This can occur at multiple levels. A reasoned debate between brother Cardinals can be brought to an immediate end should one of the two hints at accusing the other of heresy. A diocese that has proven tardy in the raising of tithes can be brought into line by the merest hint of the word. An accusation of heresy is a blunt tool, and one that can turn upon its wielder, for those accused might have previously unknown allies or patrons, and outright war between rival factions sometimes results.
There are several variant faiths within the Imperium which the Ecclesiarchy has no choice but to tolerate, even though it disagrees fundamentally with their tenets. The cults of the Adeptus Astartes are such faiths. Every Space Marine Chapter is faithful to the Emperor and its own Primarch, but they do not usually revere the Emperor as a god. Rather, to them, he is a man, albeit the greatest who ever lived. This breaks with the single most important tenet of the Imperial Creed and has on many occasions proved a source of great tension and even overt hostility between the two organizations. On the whole, however, the Adeptus Ministorum and the Adeptus Astartes try to maintain cordial relations, for the Space Marines are the literal descendants of Emperor through the blood of the Primarchs, which flows in their own veins ten thousand years after the entombment of the Master of mankind.
The Cult Mechanicus is another deviant faith with which the Ecclesiarchy is often at odds. The Tech-Priests of the Adeptus Mechanicus worship their own deity, who they call the Machine-God. As with the Imperial Creed, many sects exist within the Cult Mechanicus, and it is commonly held that the Machine-God is in fact a manifestation of the Emperor, although many in the Ecclesiarchy have great difficulty accepting this. Other sects appear to outsiders to be saturated in idolatry, worshipping the very machines they are tasked with maintaining and committing a thousand other transgressions punishable by death by the laws of the Adeptus Ministorum. Despite such differences, the Ecclesiarchy has no choice but to tolerate the Cult Mechanicus, for without the Tech-Priests the Imperium would literally grind to a halt. No institution can do without the Adeptus Mechanicus, just as none forgo the services of the Adeptus Astra Telepathica or the Navigator Houses, regardless of how distasteful they might find their servants
As has been said, while many worship the Emperor as a god, the state worship encouraged by the Ecclesiarchy is not universally practiced. For many, the apotheosis of the Emperor is not literal divinity but a deification of the state itself, a worship of the soul of humanity. There are many approaches to the Imperial Cult within the Ecclesiarchy from the rather syncretistic approach of the Amalathians to the triumphalism of the Redemptionists to the near Chaos-worship of the Xanthites.
Although the Ecclesiarchy is the largest religious body in the Imperium it is far from the only one. The most widespread non-Ecclesiarchal religion in the Empire is the Cult of the Omnissah of the Adeptus Mechanicus. There is also several religions of the Space Marine chapters which generally do not follow the Ecclesiarchy, tending to see the Emperor as the ultimate expression of humanity rather than divinity. The Averhamism of the Dark Angels is representative of the Space Marine cults, the Emperor’s ‘First and Finest’ view the Emperor as a great man and the gene father of all Space Marines. They honor Lynn El’Jonson and the Emperor as saints but they worship a single, transcendent G-d. The Dark Angels successors also practice this religion as do the Imperial subjects under their protection. In the Imperial Guard, the Imperial Commissariat is more interested in the political reliability of the Imperial Guard, Imperial Navy and Collegio Titanicus than the religious orthodoxy of the troops in their care. While the Commissariat tends towards the Amalathian viewpoint, the other religions outside the Imperial Cult that are considered licit in the Imperium are known as the Tolerati.
The Puritans are the ‘Orthodoxy’ of the Imperial Cult. They represent the mainline of Imperial religious opinion. They are opposed by the Radicals – who favor accommodating or transcending Chaos and the Imperialists who are more pragmatic in their approach seeing stability rather than uniformity of religious opinion as the key to Imperial security. The Puritans are what most people think of when you speak of the Imperial Cult.
The largest Puritan sects are Thorianity, Ardentism, the Casophilians, the Revivfactors and the Redemptionists, and the Cult of Emperor Divine.
Unlike the Puritans who focus on the Emperor as a god and the Radicals who see him as another Chaos power (though less inimical towards Humanity), the Imperialists focus on the stability and health of the Imperium as an institution of which the Immortal Emperor is merely a part.
The major Imperialists sects are the Monodominants, the Amalathians and the Anomolian Beholders.
Unlike the more conservative Puritans or Imperialists, the Radicals see Chaos as a force similar to fire. Destructive if uncontained but a powerful tool if properly harnessed. Many Imperial bodies view the Radicals as borderline – if not outright – heretics. However, the Radicals also have allies of their own in the Inquisition and the Ministorium.
The major radical sects are the Xanthites, the Horusians, the Recongregators, and the Istvaanians
The Tolerati are religions outside the Imperial cult that are tolerated in the Imperium. The most well-known is the Cult Mechanicus, others include the several Space Marine faiths such as Russism, Averhamism and Vedicism practiced by the Space Wolves, Dark Angels and Fighting Tigers respectively. The Imperial Commissariat, which tends towards an Amalathian view of the Imperial Cult, is very adept at reconciling local religion with the needs of the Imperium thus inducting new religions into the Tolerati.
Underground cults worshipping the Ruinous Powers are illegal in every world in the Imperium but that doesn’t deter the wicked or the foolish from offering praise to the Banes of Mankind. Conventional Chaos Cults or the enigmatic Cult of Is’Malalism are equally anathema to the Ecclesiarchy and to every sane Imperial subject. If you would know more about the damned, read on but know there is a price for such blasphemous knowledge.