Imperial Truth
"If once we can produce our perfect work—the Materialist Magician, the man, not using, but veritably worshipping, what he vaguely calls “Forces” while denying the existence of “spirits”—then the end of the war will be in sight."
- The Screwtape Letters
The Imperial Truth is the atheist rationalist doctrine espoused by the Emperor of Mankind as part of the Great Crusade. According to the Imperial Truth, the Emperor believed that religion and superstition had divided mankind too much during its history (which the Emperor had reason to believe, having lived through most of Terra's religious wars). When the Great Crusade left Terra, the Imperium of Man was to become a secular state espousing science and reason. Following the Horus Heresy and the Emperor's internment on the Golden Throne, the Imperial Truth collapsed, the Emperor's very own philosophy denounced as HERESY, and the Imperium of Man has become a theocratic dictatorship where questioning anything results in a painful death.
However, the Emperor didn't just ban religion simply because he disagreed with it. During his millennia of existence, the Emperor had come to partially determine the nature of the relationship sentient life had with the Warp. Knowing that the belief in gods would only serve to create them and empower the Warp creatures, the Emperor hoped to starve the Chaos Gods by denying them worshippers. What the Emperor failed to understand was that the Chaos Gods were powered not by faith, but by emotions. People going about their daily lives experience their normal emotions would still empower the Chaos Gods. What was needed was an alternative to direct the belief at, such as a God-Emperor.
When Horus had a vision of the 41st millennium, part of his instant parricidal rage was that the Emperor was abandoning the Imperial Truth by seeking to become a real god. This in turn convinced him to renounce the Emperor. And then start worshiping the Chaos Gods. Chaos is a funny thing. Of course, if he had such a vision before being infected by a chaos blade, he probably would have applied logic and observed things, such as the skulls and corpse sitting on a golden throne and realized that this is what would happen if the Emperor failed.
An alternative view is that the Emprah's plan wasn't to actual kill the Chaos Gods but to remove them from the Materium. The Emprah had a few major pet-peeves: religion, uncontrolled psykers, xenos and warp travel. Without Cultist-chan and chaos worshiping xenos, there would be no summoning of daemons. Without uncontrolled psykers, few possessions would occur. Without warp travel, there would be few opportunities for Chaos Gods to fuck Man up. Thus, without religion, xenos, psykers and warp travel, the Chaos Gods and their minions wouldn't be able to access the Materium (for the most part). Who gives a shit if the Chaos Gods are powerful if they are unable to do shit with that power? Answer, Emprah knows BEST!!!!
It might also be said that the Emprah didn't set himself up as a God for one simple reason: HE KNEW HE WASN'T ONE*. He knew Clarke's Third Law where any sufficiently advanced technology is considered by primitives as magic, and "God of the Gaps" where unexplainable phenomena are blammed away by anti-science fundamentalist barbarians as "God did it and shut the fuck up". He is also older than those four fucks who think they are Gods. When he was born there were bad bad critters in the warp that were eating human "souls", but Ragey, Plaguey and Zitpop were only just beginning to form and Rapey was still just a twinkle in the Eldar's collective eye; as such, he actually KNEW for a fact that there were no Gods and that the Chaos Gods were delusional. Admittedly, the Emprah certainly bent the truth about the "emergent warp phenomena" often enough BUT the Imperial Truth was just that: the TRUTH (hardly. Any doctrine based upon scientific materialism and skepticism doesnt work in practical terms in a universe where technolgy is animistic, prayer works, failure to burn enough witches at the stake makes your world a putrid mess with rivers of puss, Red 'Unz do go Fasta!, and the most advanced race in the settings works on prophecies and ghost powered mechanised support). It's a shame he wasn't powerful enough to destroy the big bads of Chaos until after they had "matured". Otherwise I figure he would have. Who knows, maybe he did try and got close but was just a little too late. Chaos does refer to him as the "anathema", after all.
Still, the Emprah probably would have done better to be more straightforward and say: "There are no gods, just alien dimension energy beings that are so fucked up, they think they are Gods. They are lying and will fuck you over." The Emprah's own biggest fuck up was not trusting mankind to be able to cope with the existence of eldritch entities in general without falling on our knees to worship them. Wonder how he equated that with the Cadians' daily routine of "murder the fuck out of a billion daemons". The Emprah was like a manager that hides vital information from his employees because he doesn't think they can cope with the big picture. Then, the company falls apart when an engineer pisses off a major customer because of some unknown piece of said info. That is WH40K in a nutshell.
If the Imperial Truth is even known about by the time of 40k is uncertain, although more then likely the term is now used as pro-imperial propaganda by Imperial Missionaries trying to sell worship of the Emperor to disbelieving locals on your newly discovered savage world. IN fact the novel Hammer & Anvil fall out say that's what it is.
* Note the word "wasn't" is strictly past tense, which is kinda important