Historically, various forms of Nontrinitarianism have appeared within Christianity. The term may refer to any belief about the nature of Jesus Christ that affirms God as a singular entity and rejects the doctrine of the Trinity, as affirmed by the mainstream Christianity: a consensus of Christian bishops at the First Council of Nicaea in 325. Nontrinitarianism was especially prevalent during the theological turmoils of the Protestant Reformation. A Spanish physician, Michael Servetus, studied the Bible and concluded that the concept of the Trinity, as traditionally conceived, was not biblical. His books On the Errors of the Trinity and Christianismi Restitutio caused much uproar. Servetus was eventually arrested, convicted of heresy, and burned at the stake in Geneva in 1553 under the order of John Calvin.[25]
The term Unitarian entered the English language via Henry Hedworth, who applied it to the teachings of Laelio Sozzini and the Polish Socinians. Unitarian churches were formally established in Transylvania and Poland (by the Socinians) in the second half of the 16th Century.[26] There, the first doctrines of religious freedom in Europe were established under the jurisdiction of John Sigismund, Transylvania's first Unitarian king. The early Unitarian church not only rejected the Trinity, but also the pre-existence of Christ as well as, in many cases, predestination and original sin as put forward by Augustine of Hippo, and the substitutionary atonement of Christ developed by Anselm of Canterbury and John Calvin. There were several different forms of Christology in the beginnings of the Unitarian movement; ultimately, the dominant Christology became psilanthropism: that Jesus was a man, but one with a unique relationship to God.