The Machine Cult of Mars is a part of the Imperium of Man how worship machines and builded every and all Imperium technolgy that they have recovered from the Dark Age of Technolgy. The don't worship the emperor like all other Imperium people do, but instead accept that the Emperor of Mankind as there great and noble ruler. They instead worship the advanced technogly of the past there past and a different god know as the Omnissiah (which is a big necron C'tan dragon that is inactive) how they believe completely understands tehcnolgy to the highest evolationarly point.

Heres is a page to help you understand the Adeptus Machanicus