Post Chrisitanity? Post Progress Narrative?
In general, I believe that Unitarian Universalism (though in particular I am thinking of pre-consolidation Unitarianism) constructed a post-christian identity based on Humanist Manifesto style supersessionism and the progress narrative. For a variety of reasons, the progress narrative does not really work any more. I don't think anyone has really come up with anything else to fill the vacuum left in its wake.