The Following is only my Opinion:
"Religion" is More or Less the Faith, that One/More "Beings" (God/Gods) has/have created some Stuff, like the Space-Time and the Laws of Nature and the Matter-Energy (Humans for Example), whereas "Culture" is More or Less the Knowledge, that the created Stuff, like Humans, has created some other Stuff, like Languages or Musics or juristic Laws or Architectures ...
In primitive Civilisations, the "Religion" is More or Less the Center of their Life ...
But, Step by Step, in advanced Civilisations, the Faith ("Religion") vanishes - More and More - the Knowledge ("Culture") till to the Point, in Which the created Stuff, like Humans, thinks, that They are Gods, too ...
My Point is ...
We can have a "Religion" in historical Games with primitive Civilisations, like in Rome or CK or EU and barely in Games, like Victoria or HoI ...
But, I have big Problems, to have a forced "Religion" in a futuristic Game, like Stellaris.