If you define religion to include all possible religions, even those that have few adherents, or those that have disappeared or even those not yet created, that’s not a very useful definition. I use “religion” like most people, to refer to Abrahamic religions and large eastern religions like Buddhism, Taoism, Hinduism, etc
I just mean religion in the usual sense. I've never seen the word used as a metonym until now.
I don’t think you are using it in the usual sense, as I stated, but maybe I just need an example of what you mean. I gave examples of what I think most people think of when they consider religion. Few people think of say, Athenians, to cite one of my own favorite out competed religions.
Many religions today have this feature because they out-competed religions that didn't, but it's not a universal feature of religions by a long shot. If anything, religions that have this feature are inextricably connected to social coping mechanisms(evidently due the persecution).