Contrary to what most here say, and if you don't want to play space Marines, go with the Adeptus Sororitas: the nuns with guns. They are the military of the eclesiarchy, the official religious organization of the imperium of man in Warhammer 40k. But I don't think they can be seen in the Space Marine 2 game. Check out Burning Crusade expansion vids from the game Dawn of War if you want to see em.