Holy Throne of Earth, Batman!
The place of the Ecclesiarchy and the Imperial Cult in the background of the 40kverse is absolutely solid, but it isn't often asked exactly what makes up the belief system that so dominates (and suppresses) humanity in the 41st Millennium. When it is asked, one of the most common questions seems to be what "holy books" the Imperium works on. Almost as common are questions about the liturgy of the Imperial Cult.
It was Terry Pratchett who advised me* that, if I wanted to write fantasy and science fiction, I should read history and science literature. And because I want to be as successful and author, one day, as Terry is I figured I should take that advice seriously. And because I'm also a practising Christian, one of the topics I've started to enjoy reading up on is religious and Biblical history.
"Great stuff," you say, "but is this just going to be a post about how real-life Christianity isn't as nasty and manipulative as 40k religion?"
No, it isn't. I just wanted to make sure my cards were laid clearly upon the table before I began. You see, we're used to a world where the practice of religion is dominated by the Big Four (Christianity, Islam, Judaism and Hinduism). Now, I freely admit to knowing very little about Hinduism, and I certainly hope to learn more in the future. But, for now, let me focus on the other three and say - with some confidence - that they all have holy books that guide the practice of their religion.
And yet... for all three, it wasn't always this way. Semantic, historical and archaeological analysis of the earliest documents is pretty clear that the Jewish faith - taking the oldest first - had no fixed collection of religious texts until some five hundred years BCE (or thereabouts). There were texts, of course. And there were traditional stories. But the content and emphases of these changed radically from one part of Israel to another. The Jewish scholars in the courts of Babylon were probably the first to piece together the divergent strands of their culture, partly to preserve what they perceived as a dying tradition (with the increasing influx of foreigners into Israel, intermarrying and bringing the modern, cosmopolitan gods and traditions of their lands) and no doubt partly to give authority to the delegations that returned to Israel from Babylon (c.f. the books of Nehemiah and Ezra).
We can still see the reflections of the different geographical perceptions of the One God in the various books: compare and contrast the anthropomorphic figure in early Genesis and Job with the compassionate "other" who is described in Ecclesiastes and Jonah. Look at the changing relationship between man and God in the Pslams, from start to finish (perhaps as much as three hundred years between one end and the other).
So what?
Well, before I answer that, let's give Christianity a good looking at. They've had a book pretty much from the get-go, haven't they? Well, let's overlook the first hundred years or so, before the books of what we now call the "New" Testament were brought together. For hundreds of years after that, these books and the books of the Jewish tradition (which we now call the Old Testament) were largely ignored by the theologians, scholars and doctrinists at the heads of the various sects and cults. When the Roman church became dominant, the Bible was preserved in Latin, its precise meaning obscured behind not only language and ritual but also a lack of clarity in the translation to start with. Doctrine, meanwhile, was considered to be a living thing - the Bible was history and very important and all that, but it wasn't the beginning and end of God's revelation to Mankind. Hence why the position of the Pope was considered so important: he was - officially, at least - the conduit of God's continuing revelation.
Only with the coming of Martin Luther and the Protestant movement did the text acquire - in the Roman as well as the Protestant church - the status of indisputable word of God. For centuries before that, scholars and intellectuals who could read the text had argued endlessly over its various inconsistencies, errors, contradictions and translations.
But the Muslims, surely, have got it right, haven't they? I mean, Muhammad revealed the Qur'an as a book. He was reading from a divine manuscript, so surely they had a book from the start? Didn't they?
Again, no. Certainly, once Muhammad's mission go under way there were scribes and clerks scribbling down his words so that they could be recited to those not fortunate enough to have heard them verbatim. They will have written these down on hide or bark or pottery or whatever was to hand. But the Arabic of the day was not the Arabic we see in the Qur'an. It had no diacriticals, not even the pointing system to distinguish "t" from "b" from "th". It was, largely, an aide memoire - a sort of shorthand intended to be referred to by whomever had heard the original recitation in the first place. They were using a writing system designed to record simple business transactions to encapsulate theological profundity!
To their credit, the system was kicked into shape and bashed into a proper alaphabet within two decades of Muhammad's death and the Qur'an was compiled by the second Caliph. But the question here isn't how accurate the original texts were or weren't. The point is that, once the Qur'an was compiled in full and arranged thematically so that it seemed to make a consistent sort of sense... well, it turned out not to comprise a whole religion. There were, for example, many exhortations to love God and to obey him and to submit to his will, but very few on what he actually wanted his followers to do in obedience to him. Fortunately, Muhammad himself had already been called upon to explain many of his revelations and many of those who had asked him could remember what he had said and they passed on their recollections.
Others, who had known him and worked with him were called upon for their opinion of other texts that had not been explained, or to explain and explanation that had perhaps not been as clear as it could have been... and so on. This part of Islam is called the Hadith and is the tradition of explaining how to understand the inexplicable and, often, incomprehensible.
I don't say that as a criticism of the Qur'an, incidentally. What human could hope to understand the mind of God?
So all of this brings us back to 40k. No, really, it does. Because the point I'm coming here to make is that the religion of the Imperium of Man doesn't need a holy book. Not just one, anyway. It just needs a fundamental doctrine. In Judaism this was "There is One God and we are His People". In Christianity it was "Jesus died to redeem the sins of humanity". In Islam it was "Muhammad was the Prophet of God and in his words is the key to salvation". You may argue about the semantics, but that's basically what underpinned everything else. If you believe one of those things then you subscribe to that religion.
For the Imperial Cult it is "The Emperor is a god". At the end of the day, if you - in the 40kverse - believe that, then you subscribe to the Imperial Cult. If you don't then you don't. Note that I say "subscribe to" rather than "belong to". There may well be those who believe that the Emperor is a god but that they have another god who's bigger or better than the Emperor. They don't belong to the Cult, but they subscribe to it - it has a place in their personal belief system.
Beyond this core doctrine, nothing else is required that can't be invented by its adherents. Whether the Emperor is spiritually bound to his physical remains, or whether he floats free in the Aethyr is a point of theological debate that may lead to conflict, with one side or the other holding sway at any one time. But both sides, whatever the outcome, still belong, essentially, to the Imperial Cult whatever the other side may say. Thus, books, doctrines, orthodoxies, concepts, philosophies, mythologies and miracles can all grow up as a consistent, coherent religion without needing to have a "holy book" - and certainly without needing to have a unified liturgy - at its core.
So if you want to write about the theology of the Imperial Cult, then as long as you have that core doctrinal point ("The Emperor is a god") at the heart of it, you'll be fine. Everything else is just details.
One last thing before I go: despite its name, the Imperial Cult is not a cult. What makes a cult - and I say this having given a great deal of thought to the subject - rather that a religion is that a religion provides a theological scaffolding inside which you can go ahead and build whatever sort of house you like; a cult, on the other hand, builds the house for you. The Imperial Cult barely even puts up the scaffolding. It's more like a truckload of metal poles, joints and planks inviting you to do your best.
*In an article I read that he wrote. I have not met Terry Pratchett. I once heard him speak at the Cheltenham Literary Festival, though. That was very cool.