I really believe that Unitarianism and Universalism were re-tooled in the years after the Second World War, and leading towards consolidation in 1961, to be a joint theological "other" from what had existed before. Some examples. In those early years, the Unitarian fellowship movement, heavily tinctured by materialistic humanism, got started; the Universalist General Convention was denied membership in the National Council of Churches for being insuffiently Christian (and thus leaving only one natural source for fellowship, the Unitarians); the Massachusetts Universalist Convention spawned the hell-beast known as the Charles Street Meeting House, as a field lab for a rather heady, if lyric, form of syncritism; and ministerial exchanges between the Unitarians and Universalists sealed the deal.
Its a shame nobody told the Christians, who have been acting as a rump parliament in both traditions ever since. (The organization which became the Unitarian Universalist Christian Fellowship also dates to that period. I've noticed it is among the Christians that you are more than usually likely to find a "Unitarian" or a "Universalist" and not the double moniker.)
Donald Harrington's 1960 General Assembly sermon, Unitarian Universalism: Yesterday, Today, and Tomorrowwe had a new theology, but didn't really tell us how we got there, except perhaps by de-emphasizing Christianity as past and passe. We did get a new identity, that is, Unitarian Universalism qua sect, and a rather triumphal ("America's fourth religion") and gnostic ("Are you a Unitarian without knowing it?") at that. It would be inconceivable to think that one of our own would be invited as an ecumenical delegate to a hypothetical Vatican III, but James Luther Adams was there for Vatican II. (Of course, Rome has moved too.) We're not in the American consciousness, and it isn't exactly because we were pushed off the map. We jumped.
I might believe that syncritism, as Unitarian Universalism has made it, and as I dub it "world-religionism," might work if the self-congratulating, distant, and (at times, the worst ones to be sure) xenophobic sectarianism we've inherited didn't really say that we're not serious about a theological interface from many sides.
First, it is because we have nothing like peace about God, and without God, any serious engagement with monotheism or polytheism is moot. (How many times have you heard Buddhism lauded and defended because it isn't theist?)
Second, I believe that to understand other languages you need a deep understanding of one with which to have a point of reference, and "garden variety Unitarian Universalism" (my term) doesn't allow that depth to take place.
Third, we are wed to the priviledge of class (that is, middle posing as upper) embolded by education, and overfed self-praise, and that makes it almost impossible to exhort anyone to do anything. Have you noticed that Unitarian Universalistm, for a group that's putatively interfaith, is indifferent to theological developments outside its own borders, and has no institutional will to cultivate spiritual disciplines among the rank-and-file?
I'm not sure what the best option for a future for Unitarian Universalism is, but I don't think our present trajectory is viable. I am in this dialogue to develop a set of solutions when the ears are ready to hear.
There are many reasons I became a Christian -- I wasn't one when I entered Unitarian Universalism -- but high on the list was spiritual self-preservation. Without an "other" to reach for, and lean on -- that is, God, Jesus Christ, and the Church Universal -- I would have been adrift, and out the door.