The Unitarians may hold the distinction of being the most undogmatic of all religious sects. Unitarians draw from the sixteenth-century Protestant Christian thinkers who rejected the doctrine of the Trinity and stressed the unity of God. Their outlook is sanguine: their religion is based on freedom, reason, and a belief in the goodness of human nature. The Unitarians have often been considered heretics because they want to choose their faith, not because they are rebellious.

The word “Unitarian” is a shortened version of Unitarian and Universalism. The roots of the Unitarianism beliefs were formed in Transylvania, now the border area of Hungary and Romania. The church was influenced by Ferenc David (1510–1579), himself a convicted heretic. David, a Transylvanian Nontrinitarian and Unitarian preacher, taught that prayers could not be addressed to Jesus, since Jesus was only a human and not divine. David died in prison. However, the church he founded in 1568 is the world's oldest surviving Unitarian body. His legacy of tolerance is captured in a remark often quoted by modern-day Unitarians: “We need not think alike to love alike.”

A similar Unitarian movement developed in England in the seventeenth century with a number of dissenter churches. Indeed, the early history of the movement is rife with dissension. Some members wanted to change the name to Free Christian — a title suggesting the unfettered way those members approached their vision and version of religious thought. The movement prospered and eventually became the British Unitarian Association.

The first Unitarian Church was founded in London by Theophilus Lindsey in 1773. Lindsey, like Joseph Priestley, one of the “rational dissenters” against the established church, soon encountered spirited opposition. Holding Unitarian views was a legal offense in Britain for the next forty years, until 1813, and the movement, perhaps as a consequence of the opposition, has never attracted many British adherents.

The origins of the movement in the United States developed slowly in New England out of Congregational autonomy, which stressed moderation, reason, and morals over spiritual revivalism. In 1825, following yet another schism between various sects of the movement, the American Unitarian Association was formed.

The movement stressed free use of reason in religion and believed that God existed in one person only; they did not believe in the Trinity, and, as did Ferenc David, they also denied the divinity of Jesus Christ. They had no creed and individual congregations varied widely in religious beliefs and practices. For instance, Thomas Starr King is credited with coming up with a definition that endeavored to show the difference between Unitarians and Universalists: “Universalists believe that God is too good to damn people, and the Unitarians believe that people are too good to be damned by God.”

From the early days the movement embraced the marginalized members of society, including the Universalists, in 1863 becoming the first denomination to ordain a woman to the ministry: Olympia Brown. They affirmed that God embraced everyone and that dignity and worth are innate to all people regardless of sex, color, race, or class.

The influence of Unitarianism in America is evidence by four American presidents (John Adams, John Quincy Adams, Millard Fillmore, and William Howard Taft) being Unitarians.