It's not something that *must* spread to be effective but rather a behavior principle that should influence as many features as Science, Social Growth, Politics & Economics, for example.
As a mechanic for progress - Religion sure is one of the basic foundations of Humanity's history.
Not having "it" in a civilization game is a weird contradiction at the intellectual level.
I prefer intelligent fun to *only* boring linear Kill'hem'all activities.
If they bring it back, it shouldn't be a guaranteed mechanic for progress of any kind, as it isn't like this in reality also.
Together with fitting SP-decisions like freedom of speech aso. it of course could bring progress, but together with more fascistic SP-decisions it should be what is then: A tool of oppression and reaction, which only gets numbers up together with the strong police necessary then.
But I'm not missing much, if they never bring it up.