The Imperial Cult is the official and only state religion of the Imperium of Man. It is devoted to the worship of the God-Emperor of Mankind as the ascended divinity and saviour of humanity. Those who believe in the Cult's teachings hold that the Emperor is a living spiritual entity and the only true divinity of humanity. The Imperial Cult is lead by the Adeptus Ministorum, also known as the Ecclesiarchy, who devote their lives to spreading the Word of the Emperor and doing His will. Almost all variants on the Imperial Cult are tolerated by the Ecclesiarchy's priests, but all worship of the Emperor must comply with certain Imperial values including, but not limited to: unquestioning political loyalty to the High Lords of Terra and the various agencies of the Adeptus Terra, diligence in carrying out the Emperor's will, belief in the Emperor's ultimate divinity, and conscious contributions to the Imperium's overall well-being, even if this means nothing more than helping to improve one's own homeworld or even just home city or town.
In the 41st Millennium the Imperial Cult has almost unrivaled power and influence within the Imperium. Heresy against it is punished severely. The religion is formally administered by the clerics of the Ecclesiarchy. The Imperial Cult is the Imperium's state religion, and in many ways the religion is the state itself since it is the glue that binds humanity together in the service of the Emperor and the Imperium. The precepts of the Imperial Cult, called the Imperial Creed, include the belief that all of humanity must be brought into the Imperium whereever in the galaxy humans live, the absolute abhorrence of aliens as dangers to humanity's rightful domination of the Milky Way Galaxy, and the realisation that the existence of unsanctioned psychic powers and human mutation is a dire threat to the human species which must be controlled. All of these precepts have their origins in what the Emperor himself preached during the Great Crusade of the 31st Millennium, though they ignore the Emperor's most fundamental teaching that he was not divine and that human beings should give up superstition and the trappings of organised religion in favour of scientific rationalism. Yet, because of the existence of Chaos and the true, psychic nature of the Immaterium, the Imperial Cult has ironically proven to be a powerful source of protection for humanity from the dangers presented by the Ruinous Powers.
While the ordinary citizen of the Imperium believes that the Emperor has always been venerated as the immortal and omnipotent God of humanity throughout the history of Mankind, Imperial historians and the Battle-Brothers of the Space Marine Chapters know that this was not always the case. At the beginning of the Emperor's Great Crusade in the 31st Millennium, the Imperium of Man functioned very differently. Firstly, there was no Ecclesiarchy and the veneration of the Emperor, in the form of a cult known as the Lectitio Divinitatus, was frowned upon and outright condemned by the Emperor himself. The official Imperial doctrine, known as the Imperial Truth, was that the Emperor was an extremely powerful being, the rightful ruler of all Mankind, and the perfect physical, mental and spiritual embodiment of humanity, but no matter how supreme, he was still only a human being. During the Great Crusade, however, many ordinary Imperial citizens found that the light of reason and truth brought by the Emperor was not enough to fulfill their basic human psychological desires or provide protection from the real supernatural threats that existed in the universe, and so they took to worshiping him as a deity to fulfill their spiritual needs.
During the Great Crusade, many different religious cults appeared throughout the Imperium worshiping the Emperor of Mankind as a God, each with their own subtle variations and differences. These forms of worship appeared first in those primitive planets that had regressed technologically during the Age of Strife. The numbers of these cults multiplied immensely with the Emperor's ultimate sacrifice to save Mankind from the Warmaster Horus during the Horus Heresy and his subsequent incarceration upon the Golden Throne, which was hailed as the Emperor's "ascension" back to full divinity. Most of these cults would gradually fade away, while others prospered, eventually absorbing the weaker ones. The more successful Emperor-worship cults spread their forms of worship to other planets.
The strongest of all the early Imperial cults was the Temple of the Saviour Emperor. This cult had the advantage over the others in that it was based on Terra and that its leader had been a successful and respected officer of the Imperial Guard who had fought at the Battle of Terra, defending the heart of the Imperium. This leader had re-named himself Fatidicus and had begun to preach his teachings concerning the divinity of the Emperor to anyone who would listen. This faith spread among the members of the Imperial Guard and the Imperial Navy, but also to lowly scribes and minor adepts of the Adeptus Terra (the Administratum). The faith was then spread by these individuals to other planets. When Fatidicus died at the age of 120 standard Terran years, the Temple had more than a billion followers on Terra and untold faithful throughout the Imperium's Segmentum Solar.
In the wake of the chaos and anarchy of the Horus Heresy, the Temple of the Saviour Emperor provided a message of hope and reunification through a common faith. Cults who rejected being absorbed, or who couldn't be absorbed, saw themselves being persecuted by fanatical mobs who preferred the Saviour Emperor theology. Officially, the Temple rejected this violence performed in its name. This development culminated in the 32nd Millennium, by which time almost two-thirds of the Imperium's population followed the teachings of the Temple of the Saviour Emperor, the exceptions being the Space Marines, who have never formally acknowledged the divinity of the Emperor and the Adeptus Mechanicus, who had their own form of worship in the Cult of the Machine. The Temple's importance, influence, and power rapidly outmatched any other Imperial cult dedicated to Emperor-worship.
The Emperor became an object of general religious veneration across the Imperium following the Horus Heresy and his internment within the Golden Throne on Terra. Over the decades after that conflict many individual Imperial cults sprang up throughout the Imperium, with their central theme being the redemption of humanity through the Emperor's self-sacrifice. After a few hundred years, a single cult known as the Ecclesiarchy was formed from the unification of a number of smaller cults with the Temple of the Saviour Emperor, which gradually absorbed the main body of diverse believers in the Emperor's divinity. In the 32nd Millennium this cult was recognized as the official state religion of the Imperium, was granted the official governmental title of the Adeptus Ministorum and was incorporated into the Adeptus Terra once the High Lords realized how useful the religion could be in protecting, unifying and energising the citizens of the Imperium from the myriad dangers of the galaxy. Remaining religious cults which differed in their primary theology from the Imperial Creed taught by the Ecclesiarchy were persecuted and mostly destroyed.
A few centuries later Ecclesiarch Veneris II received a seat amongst the High Lords of Terra, and after 300 years, this seat was made permanent. The political power of the Ecclesiarchy continued to grow, increasing its hold over the minds and beliefs of the Imperial citizenry. Those who wouldn't follow its teachings were declared unbelievers and heretics, ostracized, and on occasion even executed. The vast territories of the Imperium were organized into dioceses led by the Ecclesiarchy's Cardinals. These powerful religious and political figures were responsible for Missionaries and Preachers on hundreds of worlds. Lavish shrines, impressive temples, and majestic cathedrals dedicated to the God-Emperor of Mankind were built throughout the Imperium. Millions of religious pilgrims soon began making their way across the galaxy to visit particularly important religious locations, such as the world where a particular Imperial Saint had performed their most famous miracle. In time, the sheer number of pilgrims who arrived on certain worlds became an economic activity in itself and entire planets were dedicated to worship and directly ruled by the Adeptus Ministorum as shrine worlds. Particularly important shrine worlds might become the religious seat of an entire diocese and so a Cardinal would take up residence there. These planets became known as Cardinal worlds.
The only threat to the Ecclesiarchy was the Confederation of Light. Based upon the planet Dimmamar, this penitent faith's ideals of poverty and humble living clearly contradicted the teachings of the Ecclesiarchy, whose view was that sacrifices of wealth and money to the Adeptus Ministorum in taxes, tithes and other gifts were necessary to enhance the citizens of the Imperium's access to salvation and ensure that the Emperor's light reached every corner of the galaxy through his Missions. The Confederation proved too difficult for Ministorum agents to infiltrate, and the Ecclesiarchy turned to violence, supported in this effort by the unanimous vote of the Senate of the High Lords of Terra, who declared the onset of the first War of Faith, largely to ensure that Imperial stability was not damaged by the emergence of religious plurality. The entire Confederation was declared heretical and the forces of the Imperial Guard, the Imperial Navy, and thousands of fanatical zealots from the Frateris Militia were unleashed upon it, bent on its destruction. Only a few cells and hidden shrines of the Confederation managed to survive and the power of the Ecclesiarchy over the minds of men, for better or worse, was made unassailable. By the end of the 33rd Millennium every Imperial world was furnished with its own cathedral and the coffers of the Ecclesiarchy were filled with the offerings and tithes from the teeming billions of the God-Emperor's faithful. This wealth was squandered in building more and more larger and lavish churches and cathedrals and to fund Wars of Faith intended not to save the souls of humanity but to secure the Ecclesiarchy's political power and wealth.
The Age of Apostasy in the 36th Millennium was one of the most destabilizing events in Imperial history after the Horus Heresy, beginning during the long struggle between the Ecclesiarchy and the Administratum for power over the Imperium. High Lord Goge Vandire, the 361st Master of the Administratum, was a power-hungry tyrant who eventually gained direct control over the Ecclesiarchy as well as the Administratum by usurping the position of the Ecclesiarch. This made him the most powerful individual in the Imperium, and allowed him to place his own rule above even that of the Emperor. His time in power became known as the Reign of Blood, consisting of massive purges of the Ecclesiarchy, and the killings and assassinations of countless perceived traitors and conspirators. This period was eventually ended by the Ecclesiarch Sebastian Thor's new Confederation of Light, a sect of the Imperial Cult that sought to end Goge Vandire's corruption of Imperial theology. The Apostasy ultimately resulted in a major reformation of the Ecclesiarchy, the creation of the Imperial Inquisition's Ordo Hereticus to police those enemies of the Imperium who lay within its own structures and the creation of the Adepta Sororitas to serve as both the Eccelsiarchy's new military forces and the Chamber Militant of the Ordo Hereticus.
- Horus Heresy series
- Codex Imperialis, pp. 32-38
- Warhammer 40,000: Rogue Trader (1st Edition), p. 268
- Realm of Chaos: Slaves to Darkness, p. 214