Unitarianism (from Latinunitas "unity, oneness", from unus "one") is a Christian theologicalmovement named for its belief that the God in Christianity is one entity, as opposed to the Trinity (tri- from Latin tres "three") which defines God as three persons in one being; the Father, Son, and Holy Spirit. Unitarian Christians, therefore, believe that Jesus was inspired by God in his moral teachings, and he is a savior, but he was not a deity or God incarnate. Unitarianism does not constitute one single Christian denomination, but rather refers to a collection of both extant and extinct Christian groups, whether historically related to each other or not, which share a common theological concept of the oneness nature of God.
While the uncompromising theological monotheism at the heart of Christian Unitarianism distinguishes it from the major Christian denominations which subscribe to Trinitarian theology, Christian Unitarianism is analogous to the more austere monotheistic understandings of God in Judaism, and to the concept of Tawhid (oneness of God) in Islam.
Unitarianism might be considered a part of Protestantism, depending on one's stance or viewpoint, though usually it is excluded from that term due to its Nontrinitarian nature. Despite common origins during the Protestant Reformation, it is perhaps more accurate to call it a part of Nontrinitarianism. Some consider it both Protestant and Nontrinitarian, seeing no contradiction between those two terms. Nonetheless, this is not universally accepted.
Initially, the Unitarian movement was not called "Unitarian". It began almost simultaneously in the Polish–Lithuanian Commonwealth and in Transylvania in the mid-16th century. Among the adherents were a significant number of Italians who took refuge in Poland. In England, the first Unitarian Church was established in 1774 on Essex Street, London, where today's British Unitarian headquarters are still located. Since the theology was also perceived as deist, it began to attract many people from wealthy and educated backgrounds, although it was only at the late second half of the 18th century that it started to gain some wider traction within Christendom.
In the United States, Unitarianist theology spread first in New England, and 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 prayer book according to Unitarian doctrines in 1786. In J. Gordon Melton's Encyclopedia of American Religions, it is classified among "the 'liberal' family of churches".
Unitarianism is a proper noun and follows the same English usage as other theologies that have developed within a religious movement (Calvinism, Anabaptism, Adventism, Wesleyanism, Lutheranism, etc.). The term existed shortly before it became the name of a religious movement, thus occasionally it is used as a common noun that would describe any understanding of Jesus Christ that denies the Trinity or which believes that God is only one person. In that case, it would be a nontrinitarian belief system not necessarily associated with the Unitarian religious movement. For example, the Unitarian movement has never accepted the Godhood of Jesus, and therefore does not include those nontrinitarian belief systems that do, such as Oneness Pentecostalism, United Pentecostal Church International and the True Jesus Church and the writings of Michael Servetus, and which maintain that Jesus is God as a single person. Although these groups are unitarians in the common sense, they are not in the proper sense. To avoid confusion, this article is about Unitarianism as a religious movement (proper noun). For the generic form of unitarianism (the Christology), see Nontrinitarianism. Recently some religious groups have adopted the 19th-century term biblical unitarianism to distinguish their theology from Unitarianism. These likewise have no direct relation to the Unitarian movement.
The term Unitarian is sometimes applied today to those who belong to a Unitarian church but do not hold a Unitarian theological belief. In the past, the vast majority of members of Unitarian churches were Unitarians also in theology. Over time, however, some Unitarians and Unitarian Universalists moved away from the traditional Christian roots of Unitarianism. For example, in the 1890s the American Unitarian Association began to allow non-Christian and non-theistic churches and individuals to be part of their fellowship. As a result, people who held no Unitarian belief began to be called Unitarians because they were members of churches that belonged to the American Unitarian Association. After several decades, the non-theistic members outnumbered the theological Unitarians. A similar, though proportionally much smaller, phenomenon has taken place in the Unitarian churches in the United Kingdom, Canada, and other countries, which remain more theistically based. Unitarian theology, therefore, is distinguishable from the belief system of modern Unitarian and Unitarian Universalist churches and fellowships. This article includes information about Unitarianism as a theology and about the development of theologically Unitarian churches. For a more specific discussion of Unitarianism as it evolved into a pluralistic liberal religious movement, see Unitarian Universalism (and its national groups the Unitarian Universalist Association in the United States, the Canadian Unitarian Council in Canada, the General Assembly of Unitarian and Free Christian Churches in the United Kingdom, and the International Council of Unitarians and Universalists).