I feel that the overcultural model of “co-religionist” focuses on shared gods and theologies, shared belief, shared ritual structure, and shared name, and there’s an interesting and complex lensing effect involved as well that can tighten or broaden the beam. So in some contexts, I’ve seen people treating all Christians as fundamentally co-religionists, and I’ve seen people treating all Protestants as co-religionists, and I’ve seen people being concerned about finding someone of their specific denomination to marry because they didn’t want to marry someone not a co-religionist, and I keep a weather eye on the United Methodists as they slowly circle the possibility of schism because homophobia is making some of them feel like they are no longer co-religionists. (I have a lingering fondness for the Methodists, so I keep an eye on them.)
And I think that all of these things have problems in a polytheistic and pagan context, if one isn’t clear on what one is looking for in a co-religionist. “We worship the same gods” may be simultaneously true, false, and unimportant: consider a reconstructionist and a cultural-polytheist Wiccan; consider two reconstructionists who have completely different focuses within the pantheon. “We have the same theologies” likewise; it’s possible to reconcile the Heliopolitan, Hermepolitan, and Memphite cosmologies, but it takes effort, and then there’s the Greeks, with their mainline religions differing by city (most Hellenic recons go with Athens in my experience, which is also I think the mythological versions that are most familiar), the different philosophical schools on top of that, and the various mystery cults. Or even just look at how some groups of people in the ‘same religion’ have different theologies around different powers (prime powers to look at for this game are tricksters and antagonists, but that’s not the only ones that skew wildly).
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