One of the many things I value about being Unitarian is the freedom to think about religion, and explore many different spiritual traditions: Unitarianism, Universalism, humanism, Buddhism, atheism, liberal Christianity, pantheism, Paganism, Judaism, Hinduism, and so on.
Occasionally someone says, but how can you be a Unitarian and an atheist (or one of the other traditions listed above? It's simple - you love and cherish your own tradition, but you respect and value the insights of others as a corrective to any blind-spots in your own tradition. When writing sermons / addresses, my approach is to use the insights of different religious traditions to illuminate my theme; so for instance if my address was about compassion, I would draw mainly on Buddhism, Judaism and Christianity; if it was about hospitality, I would draw mainly on Heathenry and Religio Romana; if it was about the concept of a Messiah, I would draw mainly on Judaism, Christianity and Gnosticism. If my address was about religion and science, I would draw on atheist spirituality, among other things.
I count myself as a non-theist, in that I do not think the Divine is an entity with a personality; rather it is an experience or an all-pervading quality, and we can experience it through many images and archetypes. But I wholeheartedly embrace the Unitarian ethos and tradition, and many other Unitarians before me have held this view; so I do not think it makes me any less of a Unitarian.