From the video Unitarian Universalism: An Heretical History, produced by the Unitarian Universalist Church of Rockford, IL, 1995. Used with permission.
The roots of Unitarian Universalism begin at the time of Jesus of Nazareth. We sometimes forget that the earliest followers of Jesus were Jews, not Christians. They were Jewish working people who saw their leader in the context of the tradition of the Jewish prophets—Isaiah, Jeremiah, Ezekiel, Jonah, and Micah. As the influence of Jesus spread, supernatural ideas, expectations and interpretations began to be attached to people's understanding of who he was. In the gospel of Mark, Jesus is recorded as having asked his followers who people said he was, and they had a variety of responses. The book of Acts clearly records the struggle between the Jewish and Greek Christians, who it reports had no small dissention and debate because of their conflicting expectations of what it meant to be followers of Jesus.
People in the early centuries of the Christian Church held diverse ideas of who Jesus had been. Some, like the teacher Arius and his followers, believed Jesus to have been separate from the father God, while others, under the leadership of Athanasius, insisted that Jesus was, and always had been, God. And there were other views also.
The conversion of the Roman Emperor Constantine changed Christianity. Constantine who wanted to use the religion to bring unity to his empire, insisted that a council of the Christian bishops to be called to resolve for once and for all who Jesus was—God or man—and what Christians had to believe to be Christians. That Council was held in the city of Nicea in the year 325 under the protection and intimidation of Roman troops. Not surprisingly, the Council agreed with the position of the emperor's bishop and adopted what Christians know as the Nicean Creed, which affirms the teaching that God is three in one, Father, Son, and Holy Ghost, belief in what is known as the Trinity. There were those who believed that God was only one and not three. They were known as unitarians because of their insistence on the unity of God, and they were declared by the Council to be heretics.
Actually, ten years later, a Council held at Tyre restored the unitarian view as church teaching and Athanasius was exiled. And then in 381 the Council of Constantinople decided for once and for all in favor of a belief in the three-person God as central to Christianity. Throughout the years, there have, however, been Christian scholars who insisted they could find no scriptural basis for the teaching of the Trinity.
Another ancient Christian controversy was over the teaching of eternal punishment. Some Christians, like the teachers Origen and Clement of Alexandria, insisted that the loving god whom Jesus called Father would never condemn his children to an eternal hell. This teaching of universal salvation was declared a heresy in 544.
Because unitarianism and universalism were outside the mainstream and were not organized, when we trace our history we tend to include many who rebelled against the religious establishment as being spiritual ancestors, even if they weren't specifically unitarian or universalist in their beliefs, but because they were people who chose to follow their consciences.