Unitarianism is best understood in distinction from Trinitarianism. Whereas Trinitarianism teaches that God is three in one (triune), Unitarianism teaches that God is simply one (unity). Unitarianism rejects the doctrines of the Trinity and the deity of Christ.
Unitarianism should also be distinguished from Modalism, which is also non-Trinitarian. Modalism teaches that Father, Son, and Holy Spirit are three modes of existence for the One God. Modalism is found today in the United Pentecostal Church. While both Modalism and Unitarianism are non-Trinitarian, Modalists affirm the deity of Christ. Unitarians teach that Jesus was an inspired man, a great teacher, and an example to follow but certainly not God.
There have been Unitarian and non-Trinitarian heresies circulating throughout the church from the earliest ages. Arianism, which was addressed at the Council of Nicea, taught that Jesus was not fully God. Even after Nicea, there were always some aberrant groups who denied the deity of Christ and the Trinity, holding to some form of Unitarian theology.
After the Reformation, Unitarianism seemed to have a revival of sorts as various groups felt more freedom to break with official Roman Catholic (Trinitarian) doctrine. Unitarianism began to flourish in various parts of Europe. Gradually, a number of individual ministers in the Church of England embraced Unitarianism, and, later, Unitarian societies were formed. These beliefs also spread to many Congregational churches in New England in the 18th century. Harvard College, which had been founded for the training of Calvinist ministers, swung to Unitarianism with the election of a Unitarian minister to the Hollis Chair of Divinity in 1805.
Today there are a number of Unitarian groups throughout the United States and the world; probably the most well-known and influential group is the Unitarian Universalist Church. Unitarianism is usually accompanied by (or perhaps inspired by) rationalism and anti-supernaturalism. In the final analysis, Unitarians reject the specific teaching of Scripture regarding the deity of Christ and the Holy Spirit. Beyond that, they usually reject the Bible as divine revelation in favor of human reason. Horizontal relationships between human beings are considered more important than the vertical relationship between a holy God and sinful man.