Unitarian Universalism as a single tradition dates to a 1961 merger of two older, doctrinally related but institutionally separate American traditions, the Unitarians, who historically rejected the doctrine of the Trinity in favor of God's oneness, and the Universalists, who historically rejected the doctrine of eternal damnation in favor of universal salvation, a merger that combined two long standing but numerically declining liberal religious movements into a single denomination with considerably more combined institutional strength.
Because the merged tradition explicitly declines to require adherence to any single creed, framing its shared commitments instead around a set of broad principles emphasizing the inherent worth of every person, a free and responsible search for truth and meaning, and respect for the interdependent web of existence, a Unitarian Universalist minister is trained to serve congregations whose individual members may hold quite different theological views, from Christian to Buddhist to humanist to no defined theology at all, within the same congregation. This non creedal structure also means many Unitarian Universalist ministers arrive at UU ministry after training or ordination in another tradition first, whether Christian, Jewish, or otherwise, drawn to the denomination's explicit pluralism, a career path considerably less common in more doctrinally defined denominations where ministerial training and the tradition ultimately served are usually the same from the outset.