from Britannica.com

Unitarianism

 A religious movement that stresses the free use of reason in religion, holds generally that
 God exists only in one person, and denies the divinity of Christ and the doctrine of the
 Trinity.

 Theological foundations for the view of God as a unity and for the humanity of Jesus are
 found in 2nd- and 3rd-century monarchianism and in the teachings of Arius (c. 250–c. 336)
 and his followers (Arians)—both early groups of Christians whose doctrines were later
 declared heretical by the church. The modern roots of Unitarianism are traced to the
 16th-century Protestant Reformation, when certain liberal, radical, and rationalist
 reformers revived the Platonic emphasis on reason and the unity of God. Many such
 thinkers fled Italy during the Inquisition. Michael Servetus, a leading Neoplatonic Unitarian,
 fled eventually to Geneva, where he was burned at the stake by Calvinists. Some Italians
 found refuge in Poland. Chief among these was Faustus Socinus, who arrived at Kraków in
 1579. Socinus' theology stressed the complete humanity of Jesus, a view still held by most
 Unitarians and Universalists. In Transylvania, an important early figure was Ferenc Dávid,
 who was convicted as a heretic for teaching that prayers could not be addressed to Jesus
 (since Jesus was merely human). He died in prison in 1579. The church that Dávid founded
 in Transylvania is the world's oldest extant Unitarian body.

 Although some unorthodox thinkers in England drew upon Socinus and others, the
 mainstream of British Unitarianism, like that of American Unitarianism, grew out of Calvinist
 Puritanism. Calvin's doctrine of providence, coupled with an increasingly scientific view of
 the universe, led to a decline in religious orthodoxy and an increased emphasis on reason
 and morals among the more liberal Calvinist clergy. Joseph Priestley, an English scientist
 and dissenting minister, was among those who began preaching “Unitarian Christianity,”
 emphasizing Jesus' humanity, God's omnipotence, and the rational faculty of man. The
 English Unitarians became strong in Parliament, the professions, and social reform. The
 name “Free Christian” was adopted by some groups who opposed the name “Unitarian”
 as sectarian and divisive. The movement fared somewhat less well in Scotland and Ireland.

 American Unitarianism developed more slowly out of New England Congregationalist
 churches that rejected the 18th-century revival movement. Congregational autonomy
 protected from controversy those ministers who stressed moderation, reason, and morals
 over spiritual revivalism. The Transcendentalist movement of the 19th century injected
 Unitarianism with a new interest in the intuitive and emotional aspects of religion. When
 Unitarianism spread into the frontier of the Middle West, its religious fundamentals
 changed to human aspiration and scientific truth, rather than Christianity and the Bible.

 Both British and American Unitarian groups formed national associations in 1825. In 1961
 American Unitarians merged with the national organization of Universalist churches, with
 whom they shared a history of liberal idealism. In polity, most Unitarians and Universalists
 are congregational. Forms of worship, based on Protestant tradition, vary widely from
 group to group.

Universalism

 A belief in the salvation of all souls. Although Universalism has appeared at various times in
 Christian history, most notably in the works of Origen of Alexandria in the 3rd century, as
 an organized movement it had its beginnings in the United States in the middle of the 18th
 century. The Enlightenment was responsible for mitigating the sterner aspects of
 Calvinistic theology and preparing the way for the reemergence of the doctrine of universal
 salvation. The Universalists believed it impossible that a loving God would elect only a
 portion of mankind to salvation and doom the rest to eternal punishment. They insisted
 that punishment in the afterlife was for a limited period during which the soul was purified
 and prepared for eternity in the presence of God.

 The forerunner of Universalism in the United States was George De Benneville (1703–93),
 who in 1741 migrated from Europe to Pennsylvania, where he preached and practiced
 medicine. The early Universalist movement was given its greatest impetus by the
 preaching of John Murray (1741–1815), who moved from England to colonial America in
 1770. He propagated the doctrine throughout most of the colonies, often against much
 opposition from orthodox Christians who believed that Universalism would lead to
 immorality.

 The Universalism of Murray was a modified Calvinism. Near the close of the 18th century
 Universalists were to follow Hosea Ballou in rejecting Calvinistic tenets. Ballou introduced a
 Unitarian conception of God and reinterpreted the atonement: the death of Jesus was not
 a vicarious atonement for the sins of mankind but rather a demonstration of God's infinite
 and unchangeable love for his children. Ballou also put great stress on the use of reason
 in religion.

 From the 19th century, Universalists felt a close kinship with Unitarians, since the two
 groups shared many views and practices. Various attempts to unite the national bodies of
 the two denominations, the Universalist Church of America and the American Unitarian
 Association, culminated in the formation of the Unitarian Universalist Association in 1960
 and formal merger in 1961.

 Universalist churches are congregational in polity. Each church manages its own affairs but
 joins with other churches in district or regional groupings. The Unitarian Universalist
 Association consists of representatives of the local churches and the districts and seeks to
 give a continental voice to the movement. Each Universalist church is free to choose its
 own form of worship. Simple, nonliturgical services are most common, with great emphasis
 put on the sermon.

 From the beginning, Universalists have differed widely in matters of belief. Attempts to
 write statements of faith, one as late as 1935, met with only partial success. Liberalism,
 freedom of individual interpretation, tolerance of diversity, agreement on methods of
 approaching theological and church issues, and belief in the inherent dignity of man have
 been the strongest elements keeping the movement together. Universalists generally
 stress the use of reason in religion and modification of belief in the light of the discoveries
 of science. Thus, the miraculous elements of traditional Christianity are rejected as
 incompatible with modern knowledge. Jesus is considered a great teacher and an example
 worthy of imitation, but he is not held to be divine. A broader conception of Universalism
 began to emerge in the 20th century. Although stressing their ties to the Christian
 tradition, Universalists were exploring the universal elements of religion and seeking closer
 relationships with non-Christian religions.