Well, there are the Principles.
But, unless you're entirely new to this conversation (and to find this obscure corner of the blogosphere, with me trying to kindle a small fire here... it's not likely you are), you know that this is widely being discussed as insufficient.
So, I'll boldly offer an opinion or two, and see if anyone wants to argue--you'll have to bring your own coffee, I'm afraid.
We have, I submit, reached the end of one road that the Unitarians and Universalists pointed down; having abandoned creeds long ago and having displaced Christianity from a special place in the constellation of revelations, there's nothing that's easy to hand to a visitor or seeker who wants to know what it is we believe. We--as a movement, not as individuals--don't hold up Jesus as the Way any more than we hold up Buddha as the Way, or think that Lao-tze pointed to the (one) Way. So there's nothing that we've figured out that's the modern equivalent of the Winchester Profession, although it's worth noting that most of that profession is acceptable to most UUs--as long as one's willing to provide a lot of latitude about "God".
Despite being derided as insufficiently poetic (or is it sexy? I can't recall), the Principles do capture some fundamental features of our faith. Broadly depicted, there's still a universalism in these principles (although one that's lost much interest in the question of hell. Not surprising for folk who more or less concluded that they didn't believe in it); the 7th Principle affirms that we're all tied together, inextricably, and that whatever happens to us... happens to all of us, and to the rest of the universe. It's a statement that completely subsumes classical Universalist concerns; along with everything else, salvation would be something that happens to all or none. But it's more than that, since it also establishes a firm basis -- theologically speaking -- for a conservationist ethic of environmentalism.
Other principles (no, I'm not going to discourse on them all) affirm the rights and value of the individual, proclaim democratic values, and so forth. Through them, it's hard to imagine a UUism that isn't congregational or doesn't value and respect the individual.
The question keeps coming back to "what's at the core?" In the past, however perceived or thought of, Unitarians and Universalists thought of a singular deity, perhaps as Deists, and kept a hand on the Bible. So there was always the easy answer of "Christianity." I suspect that the answer was... easy.
But that's not really what those faiths were about, ultimately, nor is it what UUism is about.
What we are about is humanity. Whether there's a god or not isn't crucial; if there is, we've undertaken already -- those principles -- undertaken to take care of creation as best we can and to take care of human beings, individually and collectively, as we can. We affirm that to be... enough. Or at least, it's enough for the movement as a whole. Beyond that, we have undertaken to cradle and cherish each person's search for the truth.
Those are core values. And there's one more that's sometimes swept aside as just part of congregational polity. But it's more; our faith is truly all about what gets talked about as "Beloved Community." Community is at our core.
Sure, there are people who affirm that they're Unitarian Universalists, but who aren't part of any congregation (not even CLF), nor any of the organizations affiliated with the UUA. I don't buy that any longer. They're not UUs. They hold values and beliefs closely akin. But they're solo operators, and I think that UUs are only UUs in community.
No, that's not unique to UUism; every religion provides something of community. But what's different is that community is an adjunct to other faiths, it comes with the rest. For us, community is what lies at our core now. Over and around that, we layer the rest. Although the result can look an awful lot like traditional religion, it's not the same at all.
The same sort of conundrum exists for people who don't grasp the character of Judaism and ancient religions for which the crucial question isn't belief, but practice. People who grow up in and surrounded with faiths that practice and insist on orthodoxy, right belief, find the idea of a religion that's only tangentially concerned with right belief... and focused on orthopraxy, right practice, baffling.
UUism, however, is precisely that kind of creature. Stop asking what we believe. It's not the right question. We're not about belief; we're entirely in favor of you finding and having beliefs, but that's... secondary. Change them if you find you need to; that's great. We're about practice.
We dance up to this when we talk about "Deeds, not creeds." Then we lose sight of it and worry about what it is that we're about. Someone asks what it is we believe and we get intellectually antsy about belief. It's not belWe're about being in a community that is honest, caring, respectful and supportive. We're about being in a community that upholds, affirms and lives out the values that are in our principles.
It's about doing, being--not believing.
If someone asks you... "So what is it that Unitarian Universalists believe?" the answer is;
"We're not about what we believe; we don't even agree on what we 'believe.' We're about what we do. We share a set of values and principles, and those -- and our heritage -- call us to action, regardless of our belief, doubt, or lack of belief. We are UUs because we feel impelled to act, called to act, obliged to act--for good, for justice. We're about being and growing our community, and the community of all humanity, so that it can be, and is, a tapestry of people living in peace. We do not believe that we all need to believe alike. But we do believe that we can love alike, and that becoming that universal, loving community is what we must work to achieve. Ours is a religion that says 'You must do. You must act.'"
So... now I need to go and educate myself about the theology of orthopraxy.