Unitarian Church
 "Unitarian" can mean 'those that hold a Unitarian theological belief' and 'those that belong to a Unitarian church'. The two are very different. This is because many Unitarians and Unitarian Universalists have moved away from the traditional Christian roots of Unitarianism. In the 1890s the American Unitarian Association began to allow non-Christians and non-theistic individuals to be part of their fellowship. So people who hold no Unitarian Christian belief can be called "Unitarians," only because they are members of churches that belong to the American Unitarian Association. By 1930 the non-theistic members outnumbered the theological Unitarians not only in America but also in some Unitarian churches in the United Kingdom, Germany, Canada, and other countries. At this level it is only a liberal secular philosophy. Recent (post 1980) moves are re-strengthening the Christian religious origins of Unitarianism.

Early controversies over the nature of Christ broke out at Rome during the papacy of Victor I (189–199AD). The so-called ‘Monarchian controversy,’ originated in a revolt against the Logos theology of Justin and other apologists, who had spoken of Jesus as a second god. Such language was disturbing to some as Justin’s language seemed to promote ditheism. The view  was defended by Hippolytus and others, who thought it necessary to think of  the Father and the Logos as distinct ‘persons’.
This rejection was probably the start of "unitarian" thinking. There was a huge amount of debate, argument, division, disagreement and battle over the differences of opinion that followed on from this: the Trinitarian verses the Unitarian. The story is very long and complicated (and interesting!), too long to go into here fully. I hope we can cover it on this site sometime in the future.

Unitarianism in England
Unitarians and anti-Trinitarians were for all intents and purposes the same (this is not so today) however, at the time of the Reformation many were executed for their opinions:  Edward Wightman being burned at the stake twice (!) in 1612. In many of these cases the anti-Trinitarian sentiments seem to have come from Holland; the last two executions followed the dedication to James I of the Latin version of the Racovian Catechism in 1609. The upsurge of Socinian belief, held by men like Falkland and Chillingworth, led to attempts to outlaw Socinian books in
1640. However, the ordinance of 1648 (an act of law) made denial of the Trinity a capital offence. It was unused law: Cromwell intervened in the cases of Paul Best (1590–1657) and John Biddle (1616–1662). Biddle held several Socinian Conventicles (unlawful religious meetings) in London; as well as his own material he republished the Racovian Catechism in 1652, and the Life of Socinus in 1653.  Thomas Firmin (1632–1697), Biddle's associate and philanthropist, and a friend of Tillotson, held more Sabellian views, was inspired by Stephen Nye (1648–1719), a clergyman. Firmin wrote and published a large number of controversial tracts between 1690–1699.

The Socinian controversy, caused by Biddle and others, led to the Arian controversy of Samuel Clarke's Doctrine of the Trinity which was published in 1712. Some sixty odd years earlier John Knowles was an active Arian in Chester. Arian or quasi-Arian views abounded in the 18th century, both within the Church and among dissenters. The name "Unitarian" first appeared in 1682, being in the title of the 'Brief History' document of 1687. It was deliberatly used to describe all who, rejected the Trinity, and held the singularity of God. Firmin later founded Unitarian societies within the Church. The first Unitarian preacher was Thomas Emlyn (1663–1741) who founded a congregation in London in 1705. This was contrary to the Toleration Act of 1689, which outlawed all who preached or wrote against the Trinity. Emlyn also found himself in trouble in Dublin.

In 1689 a number of Presbyterians and Independents had merged but this came apart in 1693. Differences in administration (rather than basic doctrine) led to the
theological liberals adopting the Presbyterian name. At this time (1690–1710) many of the Presbyterian chapels fell into the hands of Congregationalists. Most of the Unitarian chapels were Independent foundations, and remained so. The liberal outlook of dissenting academies (universities and colleges) encouraged new ideas. The Salters' Hall conference of 1719 expounding the views of James Pierce (1673–1726) of Exeter, suggested letting dissenting congregations determine their own orthodoxy. The Baptists had already broken from the common doctrine. Leadership in the advocacy of a purely Humanitarian Christology came mostly from Independents, like Nathaniel Lardner (1684–1768), Caleb Fleming (1698–1779), Joseph Priestley (1733–1804) and Thomas Belsham (1750–1829). Well respected 'establishment' figures were also espousing these views: Isaac Newton was anti-Trinitarian; possibly Unitarian.

In 1773
the secession of Theophilus Lindsey (1723–1808) from the Anglican Church, (following the failure of the Feathers petition to parliament [1772] for relief from subscription) led to the formation of a distinct Unitarian denomination. Lindsey's secession was preceded in Ireland by that of William Robertson D.D. (1705–1783), who is acknowledged as "the father of Unitarian nonconformity". It was followed by other clerical secessions, mostly of men who left the ministry, and Lindsey's hope of a Unitarian movement close to or alongside the Anglican Church was frustrated. In time his type of theology superseded Arianism in the majority of dissenting congregations.

The Toleration Act was amended in 1779 by substituting belief in Scripture for belief in the Anglican (doctrinal) articles. In 1813 the penal acts against denial of the Trinity were repealed. In 1825 the British and Foreign Unitarian Association was formed as an amalgamation of three older societies, for literature (established 1791), mission work (established 1806) and civil rights (established 1818). Legal challenges were made on properties held by Unitarian Trusts and Societies dating from prior to the 1813 Penal Acts. The Wolverhampton Chapel case began in 1817; the Hewley Fund case in 1830; both were decided against the Unitarians in 1842. Appeal to parliament resulted in the Dissenters' Chapels Act of 1844: this allowed continued use as a place of public worship as long as trusts did not specify doctrines.

In the second half of the 19th century 
Unitarianism based on determinist philosophy (in the Priestley-Belsham tradition), was increasingly influenced by Channing (an American), whose works were published in numerous editions and owed a wide circulation to Robert Spears (1825–1899). Another American influence, which reduced the rigid (although limited) supernaturalism of Belsham and his successors, was that of Theodore Parker (1810–1860). In England the teaching of James Martineau (1805–1900), which was resisted at first, later became a profound influence; assisted by the influence of John James Tayler (1797–1869) and of John Hamilton Thom (1808–1894). English Unitarianism produced some remarkable scholars:  John Kenrick (1788–1877), James Yates (1789–1871), Samuel Sharpe (1799–1881) to name but three. There were few notable preachers: George Harris (1794–1859) was the exception. Ministerial training was at Manchester College, Oxford (whose origin was the academy of Richard Frankland, begun 1670 and was supported by the Unitarian Movement); the Unitarian Home Missionary College (founded in Manchester in 1854 by John Relly Beard, D.D., and William Gaskell); and the Presbyterian College, Carmarthen. There were many titles published as Unitarian periodical literature, whose influence and opinions were felt widely.Unitarianism also produced the notable English family of politicians, Chamberlain: Joseph Chamberlain, Austen Chamberlain, and Neville Chamberlain, and the Courtauld industrialist dynasty.

Unitarianism in Scotland
Famous is the execution at Edinburgh of Thomas Aikenhead in 1697, convicted of blaspheming the Trinity. The works of John Taylor, D.D. (1694–1761) on original sin and atonement had large influence in the east of Scotland, as we learn from Robert Burns; and such men as William Dalrymple, D.D. (1723–1814) and William M'Gill, D.D. (1732–1807), along with other "moderates", who were under suspicion of various heresies.  Unitarianism has always been more subtle in Scotland. The only congregation of old foundation is at Edinburgh, founded in 1776 by a secession from one of the "fellowship societies" formed by James Fraser, of Brea (1639–1699). The missionary efforts of Richard Wright (1764–1836) and George Harris (1794–1859) although enthusiastic at the time, had no lasting effect. The Scottish Unitarian Association was founded in 1813, by Thomas Southwood Smith, M.D. and others.

Unitarianism in
Debate over the Trinity exploded in Ireland following the prosecution at Dublin of Thomas Emlyn in 1703 (see English Unitarianism above), resulting in imprisonment, for rejecting the deity of Jesus Christ. Almost as a direct result of this, the Belfast Society was founded in 1705 for theological 'discussion' by Presbyterian ministers in the north, which resulted in the creation a body of opinion opposed to the Westminster standards (Anglican re-assertion of 'high' church doctrines). Toleration of dissent in Ireland was then granted in 1719 without the requirement of any doctrinal subscription. The following year a movement against subscription was established: the General Synod of Ulster, which led to the creation in 1725 of a presbytery specifically for those who advocated non-subscription, headed by John Abernethy, D.D. This Antrim presbytery was excluded by a Canonical Act in 1726 from jurisdiction, though not from communion. During the next hundred years it exercised great influence on the rest of the church and the synod; but  a mission of Scottish Seceders in 1742 produced a counter-action. The Antrim Presbytery gradually became Arian; the Southern Association of presbyteries was called the Synod of Munster and the beliefs and theology spread in varying degrees to them. By 1783 ten of the fourteen presbyteries in the General Synod had made subscription optional.  Half a century later, the synod's code of 1824 left "soundness in the faith" to be ascertained by subscription or by examination. Henry Cooke, D.D. (1788–1868) directed all his effort against this compromise, and defeated his Arian opponent, Henry Montgomery, LL.D. (1788–1865) in 1829. Montgomery led a secession which formed the Remonstrant Synod in 1830, comprising three presbyteries.
They maintained two theological chairs in Belfast until 1889, where John Scott Porter (1801–1880) pioneered biblical criticism. After 1900 Irish Unitarians sent their students to England for their theological education, though in certain respects their views and practices remained more conservative than those of their English brethren.

In 1910 the Antrim Presbytery, Remonstrant Synod and Synod of Munster united as the General Synod of the Non-subscribing Presbyterian Church of Ireland, with 38 congregations and some mission stations.

Unitarianism in
United States
Unitarianism in the United States development similarly to England, and passed through the stages of Arminianism, Arianism, to rationalism and a modernism based on an open-minded acceptance of the results of the comparative study of all religions. In the early 18th century Arminianism presented itself in New England, East Coast states, and sporadically elsewhere. This tendency was largely accelerated by a revolt against the "Great Awakening" of protestant evangelical activity under Jonathan Edwards and George Whitefield. Before the War of Independence Arianism showed itself in individual instances, and French influences were widespread in the direction of deism, though they were not organized into any definite doctrines by religious bodies. Unitarianism developed steadily afterwards.

Unitarianism in other Countries

Many other countries had strong traditions and long histories of, Unitarianism: Easter European Countries, in particular Poland and Hungary. A number of enclaves exist in Europe. Countries which were exposed to Islam had a tendancy towards Unitarian thinking.

Unitarian Christian Principles of faith
There are essentially two strands:
Biblical (or Evangelical) Unitarians hold similar beliefs to other evangelical Christians: apart from their rejection of the Trinity.

Liberal Unitarian Christians (also called "Unitarian Christians") do not believe in the Trinity, the Virgin Birth, the deity of Christ, or Biblical infallibility.

Neither group have a creed or formal statements of faith that must be believed in order to be accepted into membership, or fellowship with other Unitarian Societies. However, they have set out some basic principles that distinguish their faith from other Christian groups:

the belief in One God and the oneness or unity of God. the life and teachings of Jesus Christ is the exemplar model for living ones' own life. that reason, rational thought, science, and philosophy together with religion and faith are not mutually exclusive. that man has the ability to exercise free will in a responsible, constructive and ethical manner with the assistance of religion. the belief that human nature in its present condition is neither inherently corrupt nor depraved, but capable of both good and evil, as God intended. the conviction that no religion can claim an absolute monopoly on the Holy Spirit or theological truth. the belief that the works of the Bible are inspired by God, but were written and edited by humans and therefore subject to human error. the rejection of traditional doctrines that they believe malign God's character or veil the true nature and mission of Jesus Christ, such as the doctrine of predestination, eternal damnation, the Trinity, and the vicarious sacrifice or satisfaction theory of the Atonement.
Most Unitarian Christians say that Jesus, his followers and disciples would today be defined as Unitarian Christians, and that Unitarian Christianity is the form of Christianity most closely following the direct teachings of Jesus. However, Unitarian Christians respect the beliefs of others and do not believe that the Unitarian Christian way is the only way to follow God's will. Unitarian Christians believe Jesus did not claim to be God nor did his teachings hint at his divinity or the existence of a triune God. Unitarian Christians generally do not believe that Jesus was conceived in the womb of a virgin. Some do not believe Jesus performed miracles to the extent reported in the Gospels. Unitarian Christians give the most weight regarding the accounts of Jesus, his character, and his life to the four canonical Gospels (Mark, Mathew, Luke, and John). Most also accept other sources of information about Jesus including newly discovered Gospels that were not included in the original canon of the Bible (e.g. Nag Hammadi Library).

Unitarian Christians reject the doctrine of some Christian denominations that God chooses to redeem or save only those certain individuals that accept the creeds of, or affiliate with a specific Church or religion, or corruption of the mass of humanity. They generally do not believe that God demands belief in certain principles of faith and that no good works in life are required to be morally righteous.

Most contemporary Unitarian Christians believe that the mixing of state, politics and religion is not acceptable: one's personal moral convictions will guide their political activities and a secular society is the most viable, just, and fair society.

Groups and Associations
Unitarianism developed in the 1600s during the Protestant era of the evolution of the Christian church. Unitarian strongholds arose in Eastern Europe, Britain and the North Eastern parts of the United States. Unitarianism was independant and in the congregational tradition: each congregation governed itself. These congregations did recognise and worked with other similar congregations, joining together as formal associations of churches. The largest and most influencial of these are The American Unitarian Association, formed in 1825. The Unitarian Universalist Association (UUA) formed in 1961 when the American Unitarian Association and the Universalist Church of America merged forming the largest organization of Unitarians in the USA. The UUA is no longer an explicitly Christian organization and does not focus on the core teachings of Jesus Christ or Christianity.

Several Unitarian organizations still hold Christianity as their central theme including the Unitarian Universalist Christian Fellowship (UUCF, an affiliate of the UUA), the General Assembly of Unitarian and Free Christian Churches (GAUFCC) of the United Kingdom, and the Unitarian Christian Association (UCA, an affiliate of the GAUFCC). The American Unitarian Conference (AUC), formed in 2000; it's stated goal is to 'formulate and promote classical Unitarian-based, unifying religious convictions, which balance the needs of members with a practical approach to inclusion and progressive free thought'.

Ecumenical Relations
Unitarian belief almost always means severance with Christianity as understood by the Nicene-Chalcedonian churches (Anglican, Orthodox, Roman Catholic, and most Protestants). Unitarianism is outside these traditions, it has a tradition of its own: Unitarianism, parallel to but different from Trinitarianism. Anglicans, Catholics, Orthodox and Protestants insist on Trinitarian belief as an essential doctrine of Christianity, and necessary to continuity with the historical Christian faith.

Occasionally, especially among Protestants, Trinitarian groups have grown friendly to or incorporated Unitarianism. Friendliness toward unitarianism has sometimes gone alongside anti-Catholicism. In some churches non-trinitarian or unitarian belief has been partially embraced, and tolerated by others for 'pragmatic reasons'. This was the case in the English Presbyterian Church, and in the (American) Congregational Church in East Coast states late in the 18th century. The Restoration Movement later attempted to create a compatible relation between Trinitarians and Unitarians. The Seventh Day Baptists, Adventists and some Apostolic Faith Churches succeeded in this. The Unitarian tendency in these denominations is likely due to intrinsic skepticism about Catholism as a reliable guide to Christian tradition and doctrine.

In some denominations, this openness to Unitarianism within traditionally Trinitarian churches has been inspired by ecumenical motives. There are cases too, where some denominations have been accused (from within and without) of abandoning their standards and beliefs to work alongside Unitarians. The history of Unitarianism is one of division and controversy: in some quarters it still is.

General Assembly of Unitarian & Free Christian Churches (UK)
Unitarian College Manchester (UK)
Unitarian History Society (UK)
Unitarian Societies (UK)
Scottish Unitarian Fellowship (UK)
Scottish Unitarian Association (UK)
Unitarian Universalist Association of Congregations (USA)
First Unitarian Church (USA)