Traditionally, Unitarianism was a form of 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 the early Christian Churches emerged in late antiquity, one by one they cut themselves off from the Unitarian Mosaic Clergy. The first to do so was Sylvester the Bishop of Rome in 318CE followed by Emperor Constantine when Unitarianism was rebuffed by orthodox Christianity at the First Council of Nicaea in 325. Constantine passed such harsh legislation against them in 333CE that in 345CE many were forced to emigrate to Kerala. The others established their base in Nisibis. The Nestorian Church first distinguished the Unitarian Mosaic Clergy as Apostates (Hunefa) during the reign of Khosrau II Parviz (22nd Sassanid King of Persia 590-628CE) for rejecting the election of Nestorian Catholicos Gregorius of Seleuciain 604CE. Gregory responded against the Unitarian Mosaic Clergy by sending Babai the Great in 604CE who evicted them from the Monastery of mount Izla in Nisibis driving them into the Arabian desert.
In the desert they found refuge with Nestorian converts among the Quraish (the most famous of whom at this time being Waraka ibn Naufal and his cousin Khadijah and her 34 year old husband) who also rejected the election of Gregory as Catholicos. Here, in 610CE Abu Qasim at age 40 was established over them as His Grace "Mahomed" ("Most Graceful") Bishop Mustapha ibn Abdullah, and many other Nestorians like Salmān e Pārsī joined the protest movement in the desert.
Unitarian churches were formally established in Transylvania and Poland among the Khavars (by the Socinianism) in the 16th Century. Michael Servetus, a Spanish proto-Unitarian, was burned at the stake in Geneva, in 1553, on the orders of John Calvin.
