Unitarians, struggling in the UK with dwindling congregations, cannot understand how thirty or so congregations Sunday Assembly can come from nowhere and steal the thunder (or steal the drizzle) from the hard effort that makes a Unitarian congregation just tick along.
Sunday Assembly is the atheist answer to church, but it doesn't discuss God. It has uplifting pop songs providing singalongs, and uplifting readings. God isn't involved, not even discussed in the negative.
First of all, the Sunday Assembly is in take off mode.Take off is not sustainability. It's what happens when something is novel, different, a cost to set up but all go at launch. The fact is that there used to be Ethical Churches and they all died except Conway Hall in London, and that ceased to be a church of any kind. The Ethical Churches contained liturgies that were humanist in assumption, like Liverpool's strange Book of Common Prayer type material that celebrated life and effort and realised not all that happened was celebratory. I still use and adapt the material I borrowed.
I could provide a service where we all sing along to pop songs instead of hymns. But I'd want to make reference to spirituality and to traditions of insight that are religious and spiritual, even though I think God is a human construction, even with the possibility (only) of transcendence. These traditions give insight. I don't want to ignore them.
On their own, most of these narratives are pretty dead, I'll accept. I don't believe in an alternative history of the world in the sense of a fall from which we need saving. We don't need saving, nor does the world need redeeming. Saviours do not visit, guide or have messages from God. We are what we are, and it is what it is, and it will die when the sun is too big. But beyond the alternative cosmologies, there are reflections, and these traditions can contribute. These are not just museum outings either, but artefacts that are artistic today.
And I say that whilst being more of a realist than I was, about science, social science, and research. If someone discovers the Buddha wasn't like the tradition, or that there is no archaeology behind the supposed origins of Islam (until it has 'arrived' at the Middle East), then that matters and is NOT 'just another story' as the more dedicated postmoderns would insist. But religion is like art, and its researches tend to be from other disciplines, and is closer to stories and fictions. There isn't a historical or scientific method into resurrection or virgin births (and plenty against). Mythology is mythology no matter where it comes from, but myth isn't value-free and value-useless. The truth of the world includes its art.
I don't think Sunday Assembly can stay where it is. If it does these things to also have people support one another in congregations, then inevitably they as congregations will have to discuss human values and human failings. There'll have to be some explicit philosophising or even humanistic theologising. The joke that 'West Coast Unitarians [of the USA] gather to discuss their tax returns' has a comparative truth to it but isn't true.
Here's another, practical, problem. Music used and played within divine worship is not subject to copyright regarding performance. So all the music I put to CD and use, and store in the church for potential use and as a record of use, is copyright free. Nothing is ever sold, either. I'm not sure if the Sunday Assembly can claim equal divine use, that is something it specifically denies. The law is an ass on this, as no one can identify what is divine, and it ought to be congregational and reflective. But Unitarians can claim it and they, presumably, cannot.
Sunday Assembly may work. Personally, I doubt it; in the UK, Unitarianism barely works - it is about freedom of difference worshipping together, and is about the mixing of the liberal Christian, the westernised East, the religious humanist, and the romantic with the rational. Some understand its individualism, but more ask 'what's the point?' and the British don't like joining clubs. If Sunday Assembly works, then good for it, and if it doesn't then one or two may come to the Unitarians. The effect on Unitarianism by copying or taking in folk might be a touch of the charismatic and a further push towards common narratives. It wouldn't do it any harm.