Who’s gonna ban it? The government?

Anarchists should fight against all oppression, including religious oppression. But any religion, spiritual community, or other related sort of thing, if they aren’t oppressing anyone (and I figure “not oppressing anyone” is basically what you meant when you stuck “voluntary” in your post body), why TF would you go and make it your business what they do?

Beyond that, one could argue that without being able to oppress people, religions would die out over time as people wake up to what they really are. To the extent that that happens, great. But I do think there’s something deep in humans that has spiritual sort of leanings. Whether what remains once oppressive religions are gone will qualify as a “religion” or be more of a set of secular spiritual practices or some such really depends more on your definition of “religion” than anything else.