Though still in the drafting stages I am working on running a Dark Heresy game (now be warned - I as a GM feel free to steal anything I can get my hands on then twist to my own evil means....)
My game is going to take place in small cluster of stars on the edge of the Calixas sector called the Sword Worlds (stolen: Traveller) where there are three suns with multiple planets with Sword names (get it?) in close proximity to each other.
The area was liberated from Chaos in the Sword Crusade a few millenia ago and now has an Space Marine presence (Black Templars) and a Sisters of Battle presense - the Eclesiarchy started the crusade but the Black Templars had to finish it.
Just back ground stuff - more but unnecessary.
The real secret is that at the end of the Dark Age of Technology where worlds got separated there were still human religions around (such as Christianity and Judaish, Hindu and Muslim etc...) but those all were "purged" during the Emperor's Cruade (I kind of gather that the Emperor was not a huge fan of religion so I had him try to wipe it out before the Horus Heresy).
In the Sword Worlds which was cut off and had to be reintergrated during the Crusade - Christianity still flurished until the Cruaders arrived and reintergrated them into the Imperium. Christianity went underground.
During the Horus Heresy and the rise of the Imperial Cult afterwards this sect remained hidden. Eventully the Sword world fell to Chaos or Heresy (not sure which) and had to be re-reintergrated as stated above.
As my PCs adventure I plan to have them stumble on this secret sect of Christians who deny the Emperor and have have thier own secret military (a modern Knights Templar or something).
Just looking for any thoughts or ideas or just to generate a discussion.
Todd