There has been much debate recently about whether Unitarianism (and Unitarian Universalism) is Christian, post-Christian, universalist (in either the modern sense or the 19th century sense), or something else.
In our insistence on being non-creedal, have we adopted an "anything goes" approach?
Many Christians would claim that Unitarianism is not Christian, because most Unitarians do not believe in the Trinity and the doctrine of vicarious atonement. But not all Christians believe in vicarious atonement (at least not in the penal substitution theology version of it). The Orthodox Christian view of Christ's role and function is quite different from that of Western Christians. Early Christians did not believe in the Trinity; the doctrine was finalised at the Council of Nicaea in 325 CE. So neither of these beliefs are essential to Christianity. It might be argued that certain values (such as forgiveness, social justice and compassion) are unique to Christianity, or promoted by Christianity more than by other faiths - but in fact Roman polytheism listed compassion as a virtue. It is difficult to define any religion by listing its beliefs and values, because religion is about identity and community rather than beliefs and values.
Many Unitarians would claim that Unitarianism is not Christian, sometimes for the same reasons that some Christians would make that claim, and sometimes because Christianity is viewed as exclusivist (in the sense of regarding itself as the sole possessor of truth) - but many Christians take part in interfaith dialogue and study other faiths in a spirit of humility which avoids cultural and theological imperialism.
My own view is that Unitarianism has Christian roots (and that the tree is made of the same wood). We use the Bible in our services. We are rooted in a Christian culture (whether you like it or not, Western Europe has been Christian for centuries - and yes, that religion was imposed by the sword, but it's still part of our culture, rather like the way that British law and morality is part of the culture in post-colonial countries). The values promoted by Christianity (and by other traditions) are still widely valued in our culture, and by Unitarians. We still celebrate Christian festivals, together with all the Pagan trappings that come with them (who doesn't like Christmas presents, Easter eggs, Christmas trees, and all that?) We still think Jesus was a good bloke.
And yet... Unitarianism and Unitarian Universalism include some unique traditions of our own: Transcendentalism, Deism, Universalism (in both senses), the Flower Communion, the Water Communion, the flaming chalice. We have the writings of our Unitarian and Universalist forebears. And, inherited from Servetus, some Unitarians have a pantheist understanding of the Divine that was partially informed by Neoplatonic and other hermetic writings, which has fed into the Pagan revival via Ralph Waldo Emerson and other Transcendentalists, and made room for Pagan spirituality in the 20th and 21st century.
So it is true that Unitarianism is both Christian and non-Christian, depending on your understanding of what "Christian" means. Given that mainstream Christians can't agree on what it means, it's hardly surprising that Unitarians can't either.
I think the key to defining whether a group or an individual fits into any particular category is, in the end, about membership and identity. What do you identify as - and do the other people with that identity agree with your self-identification?
I do not identify as a Christian, but I do identify as a Unitarian, because I am accepted as a member of a Unitarian community; I share the views of the majority of Unitarians about the value of the Bible and the Christian tradition; I espouse the values of Unitarianism; and I am pretty well versed in the history and culture of Unitarianism.
But there's nothing to stop a Unitarian from identifying as a Christian. Unitarianism does after all emerge from the Christian tradition, and there's much to value in the Christian tradition (as well as much to criticise). And if we reject the Christian tradition outright, we reject much that is of value.