Omnissiah

In Warhammer 40,000, things are so grimdark that people have forgotten all of the science that was once known; the only remaining knowledge is encoded in religious ceremonies followed by the Adeptus Mechanicus. The deity central to mechanicus dogma is the Machine God, who is also known as the Omnissiah.

History of the Machine

The rise of the machine cult is hidden in the history although it is known when the Emperor of Mankind began his great crusade Mars was dedicated to the Omnissiah. Mars was a great power, with unrivalled technology and secrets. If he was going to launch his great crusade the Emperor needed Mars support although this put him in a pickle, because of his Imperial Truth which said no gods or spirits or anything supernatural truly existed. This of course ruled out the Omnissiah as well but the Emperor like any good politician decided to quietly brush it under the carpet and declared the techpriests were a one-off exception to humour. Likewise, to humour their population, the techpriests declared the Emperor was a part and aspect of their God, which you can imagine he didn't like very much but again he grinned and beared it.

Current times

Today the cult of the Omnissiah is one of the state religions of the Imperium, not matter how much the Ecclesiarchy fumes about it. The forge worlds are the centre of his worship although each regular Imperial citizen knows of the machine god and at least respects him for his secrets of technology he gave mankind. Usually he is sold as an aspect of the Emperor as noted above.

Theories of true identity

While this may seem straight forward, the actual identity of Omnissiah has become the greatest game of 'Guess Who?' in 40k, as several theories of his true form have now come forward, which are covered below.

Theory One: God-Emperor of Mankind

Some tech-priests believe that the God-Emperor of Mankind is the Omnissiah, or at least a form of him. These include the tech-priests who remained loyal to the Imperium of Man during the Horus Heresy. The Adeptus Mechanicus officially holds this view, and it is used to pacify the more zealous member of the Ecclesiarchy and Inquisition who doubt the Mechanicum's loyalty, though there are many individuals who believe they are separate. This logically though does lead to causes of conflict between the two bodies and it is something that will never truly be satisfied by either side.

Theory Two: Void Dragon

Other tech-priests equate the Omnissiah with the Void Dragon, one of the Star Gods of the Necrons. These tech-priests also tend to venerate the Necrons as the Omnissiah's angels. They then tend to do stupid things like poking around in Necron tombs, trying to probe them for secrets as part of their "Quest For Knowledge." They tend not to live very long, as Necrons don't care what any life-forms think of them; they are equal-opportunity destroyers. In the recent Horus Heresy fluff, it was shown the Emperor sealed a silver dragon under Mars in the distant past in order so humanity could have control over machines. Somehow he was able to make it so humans could take some of the dragon's power then for their own although in hindsight putting a super powerful creature in the same solar system as the capital of the Imperium might come back to bite them in the ass...

Theory three: Both

Some versions of the myth tend to separate them in two, with the Void Dragon being the Machine God, and the Emperor being the Omnissiah who "tamed" him. It's unclear, really

Theory four: Chaos Undivided

The Dark Mechanicus, those tech-priests who sided with the Chaos Space Marines during the Horus Heresy, believe that the Omnissiah is a combined aspect of Chaos Undivided, worshiping Chaos as a form of progress and knowledge. Said tech-priests tend to take the mechanical augmentations a little further than one would like in a tech-priest. They also use bio-mechanics, which is very explicitly forbidden.

Theory five: Individual warp entity

It could be over time that like any other strong collective desire and belief the tech-priests have created their own God in the warp, which would be a Machine God of logic, machines and arcane processes. While he may have started off as one of the other theories, such belief just doesn't do anything and proof of previous miracles caused by strong belief in the god and his servants the machine spirits have been seen.

Other

Presumably, any sufficiently-advanced machine will be taken for the Machine God by at least some tech-priests, like a functional Man of Iron, or other Dark Age technology. They could for example worship a rediscovered ipod as a significant avatar of his machineness.