A religion is a method or set of methods by which humans manage our relationship with the spiritual, or the numinous, or whatever term one likes (even ‘God’… not a term I use). Religions arise through a folk-process, like styles of music or art. Like those, major practitioners can shape practice or invent ‘schools’ of practice. In religious matters those often become more easily identifiable as ‘religions’. However most traditional religions don’t have ‘founders’, but rather grow out of the soil of their originating culture.
People join religions when they actively take up the practices of those religions, and play with approaching the world through the assumptions of related philosophies. Neopaganisms have been that way, growing out of the culture of the late-20th c post-modern ‘West’. That said, ‘Churches’ are organizations created to promote the practice of a religion or religious tradition. Most churches do not contain all the members of the religion it practices – that’s kinda the mark of a ‘cult’, perhaps. Even the Roman Catholic Church is far from the only organization working the religion of Western Sacramental Orthodox Christianity (if you will… Christianity is complicated). There are numerous unrecognized apostolic lines, funny little garage-chapels, etc, across the world.
A church is a religion-club; a group of interested, even passionate folks who want to pursue a spiritual work. People join churches because they are identified with or interested in the related religion, or because of their neighbors, or families. Like any club, just the interest in the topic may or may not be enough to bind a group of strangers or acquaintances into a creative circle. The same problems that attend a local arts group or little-league will exist for attempted churches. Rules, by-laws, budgets… there’s more work in doing a Church than just having a religion, and no degree of spiritual growth makes people suitable to keep the books.
Joining a religion-club is not the same as joining a religion, nor is leaving the club equivalent to leaving the religion. You cannot ‘become a Druid’ by paying your annual donation to ADF Inc. (my home club) You participate in Druid religion by working Druidic spiritual methods for yourself. ADF has placed our most recent basic instructions for working the religion outside our paywall as our Hearthkeeper’s Way instruction. Someone recently called it the ‘D&D Starter-box’ for a home Pagan religion. Those looking for a walk-in guide to establishing a Pagan practice should have a look.
I suppose, then, that we expect (even hope) that our style of Paganism will be available as religion regardless of individual membership in our club. Conversely since we are the largest (though not only) fellowship for this style of Paganism we hope that by generating more members of our religion we can, in turn, recruit members for the club.
But if someone takes up the work, creates a shrine, offers to the Kindreds, studies the Old Ways and makes an effort to live by virtue, they are practicing our religion, whether they ever join our fellowship, or not.
No big wrap-up, except maybe to say that joining or leaving a church should not generally be equated with joining or leaving a religion. The choices are separate.