Unitarianism (from Latin unitas "unity, oneness", from unus "one") is a Non-trinitarian Christian theological movement that believes that the God in Christianity is one singular entity, as opposed to a Trinity (tri- from Latin tres "three"). Most other branches of Christianity define God as one being in three persons: the Father, Son, and Holy Spirit.[1] Unitarian Christians believe that Jesus was inspired by God in his moral teachings, and he is a savior,[2][3] but he was not a deity or God incarnate.

Unitarianism is also known for the rejection of several other Nicene Christian doctrines,[4] including the doctrines of original sin, predestination,[5][6] and the infallibility of the Bible.[7] In J. Gordon Melton's Encyclopedia of American Religions, the Unitarian tradition is classified among "the 'liberal' family of churches".[8] Unitarians place emphasis on the ultimate role of reason in interpreting scriptures, and thus freedom of conscience and freedom of speech in the pulpit are core values in the tradition.

The movement is proximate to the radical reformation; Unitarianism began almost simultaneously among the Polish Brethren in the Polish–Lithuanian Commonwealth and in Transylvania in the mid-16th century; the Christian denomination that emerged is known as the Unitarian Church of Transylvania. Among the adherents were a significant number of Italians who took refuge in Poland.[9][10] In the 17th century, significant repression in Poland led many Unitarians to flee or be killed for their faith, notably Katarzyna Weiglowa. From the 16th to 18th centuries, Unitarians in Britain often faced significant political persecution, including John Biddle, Mary Wollstonecraft, and Theophilus Lindsey. In England, the first Unitarian Church was established in 1774 on Essex Street, London,[11] where today's British Unitarian headquarters is still located.[12]

As is typical of dissenters, Unitarianism does not constitute one single Christian denomination, but rather refers to a collection of both existing and extinct Christian groups, whether historically related to each other or not, which share a common theological concept of the oneness nature of God. Unitarian communities have developed in Britain, Ireland, South Africa, Central Europe, India, Canada, the United States, Jamaica, Nigeria, and Japan.

In the United States, different schools of Unitarian theology first spread in New England and the mid-Atlantic states. The first official acceptance of the Unitarian faith on the part of a congregation in America was by King's Chapel in Boston, from where James Freeman began teaching Unitarian doctrine in 1784, and was appointed rector and revised the Book of Common Prayer according to Unitarian doctrines in 1786.[13]