What differentiates an "atheist church" from an Epicurean "garden" if the church-goers don't accept any supernatural causes? What do "we" offer as a distinct philosophy to secular "nones"?
We have the philosophy but not the community. I've no experience with atheist churches, so for me a question would be "what do they offer?"
Of interest is the community and morality offered by any church, atheist or theist or deist or whatever. I think that to some degree these are as important as the theology, both to unite the congregation and to separate them from outsiders. Note all of the people born into a given church who disregard the supernatural but still follow the morality established, supposedly, by that supernatural. There is also the "advantage" of being able to follow a pre-established morality and so not having to engage with difficult questions to the degree, perhaps, of somebody following a relative morality. In an atheist church there is the danger of settling upon an absolute morality; a garden avoids that. But I think that, throughout time, community and morality are at least important as the supernatural for the allure of churches, and at least as divisive in terms of an overall culture or "us" v "them."