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 central figure in the tech-priests' religion is the being known as the Omnissiah, also known as "the Machine God."

God-Emperor of Mankind

Some tech-priests think 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.

Void Dragon

Other tech-priests worship 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.

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.

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.