Unitarianism, in general, the form of Christianity that denies the doctrine of the Trinity, believing that God exists only in one person. While there were previous antitrinitarian movements in the early Christian Church, like Arianism and Monarchianism, modern Unitarianism originated in the period of the Protestant Reformation. In Geneva, Michael Servetus was burned at the stake (1553) for his antitrinitarian views. Under Faustus Socinus a strong center of Unitarian belief developed in Poland. In Transylvania, Francis Dávid laid the foundation (c.1560) for the Unitarian Church there. In the 17th and 18th cent. Socinian ideas took root in England, especially under the influence of John Biddle, called the father of English Unitarianism. The development of a separate Unitarian body came about gradually through the efforts of such men as Joseph Priestley and Thomas Belsham. Originally a scripturally oriented movement, in the mid-19th cent. Unitarianism became a religion of reason under the leadership of James Martineau in England and Ralph Waldo Emerson and Theodore Parker in the United States. Reason and conscience were considered the only guides to religious truth; complete religious toleration, innate human goodness, and universal salvation were preached. Unitarianism took hold in the liberal wing of the Congregational churches of New England. At King's Chapel, Boston, in 1785, trinitarian doctrines were removed from the liturgy. In 1796, Priestley, who had fled to America to escape persecution, established a Unitarian church in Philadelphia. Liberal Congregationalists in New England gradually formed themselves into a new denomination, to which the name Unitarian was given (c.1815) by their conservative opponents. The final separation from Congregationalism was hastened by the choice of Henry Ware (1764–1845), a liberal, as Hollis Professor of Divinity at Harvard in 1805 and by the ordination sermon defending the liberals preached (1819) by William Ellery Channing in Baltimore. Channing's statement of Unitarian beliefs became the platform of the denomination. The American Unitarian Association was formed in 1825, and in 1865 a national conference was organized. A congregational form of government prevails in the Unitarian churches, each congregation having control of its own affairs. Neither ministers nor members are required to make profession of any particular doctrine, and no creed has been adopted by the church. The covenant in general use is simply, "In the love of truth, and in the spirit of Jesus, we unite for worship of God and the service of man." In 1961 the Universalist Church of America merged with the American Unitarian Association to form the Unitarian Universalist Association.
See J. F. Clarke, Manual of Unitarian Belief (20th ed. rev. 1924); D. W. Howe, The Unitarian Conscience (1970); S. E. Almstrom and J. S. Carey, ed., An American Reformation: A Documentary History of Unitarian Christianity (1984); D. Robinson, The Unitarians and the Universalists (1985).