In previous posts, I have looked at whether Unitarianism is Christian or not, and what kind of Christianity it is heir to. In my view, it is the heir of Christian heresies. In the ancient world, a heresy was a school of thought, and not a pejorative term implying deviance from the true path.
Some of the earliest Christian heretics were the Ebionites, who were the Church in Jerusalem. They were led by James, the brother of Jesus, who probably knew a thing or two about Jesus' life that did not get into the Gospels. The Ebionites took their name from the Hebrew word for poor; this was derived from the many references to the poor in the Hebrew bible, and from the line in the Beatitudes, "Blessed are the poor in spirit" (which I always assumed meant that the poor were spiritually blessed, not that the poor-in-spirit were blessed). They did not believe in the doctrine of the virgin birth, which was in any case based on a mistranslation of the Hebrew word for a girl or young woman into Greek (parthenos). They also rejected the teachings of Paul of Tarsus. They may have been vegetarians, and they held an adoptionist view of Jesus as the Messiah. They are also said to have denied the pre-existence of Christ, and vicarious atonement (both doctrines which Unitarians have rejected since the Racovian Catechism).
Some Unitarians look to the Gnostics as spiritual fore-runners, but since the Gnostics were world-denying and Unitarianism is world-affirming, I personally do not think that the comparison is helpful, except insofar as the Gnostics denied vicarious atonement, and held that the purpose of Jesus' existence was to bring enlightenment.
Other early Christian groups which broke away from the mainstream were the Miaphysites and the Dyophysites. They rejected the compromise on the nature of Christ which was hammered out at the Council of Chalcedon in 451.
The Pelagian heresy is interesting, in that Pelagius rejected the doctrine of original sin. However, the implication of Pelagianism is that since humans are capable of exercising their free will to become good, we ought to be able to live blameless lives, and there is no excuse for not doing so. Pelagius himself was quite puritanical, and expected people to live up to quite high standards. Semipelagianism sounds more manageable than full Pelagianism.
The Arian heresy (see my previous post) was a very important and widespread view, which influenced early Socinians. Arianism was very popular in the early medieval Germanic kingdoms, and for a while, it looked as if it would become the mainstream theology of the church in Northern Europe.
The Racovian Catechism was an important milestone in the development of Socinian thought, though it is by no means the final statement of either Socinian or Unitarian theology, and it was recognised by its authors that it was a provisional statement, and that the church was open to the discovery of new truth.
Why are all these ideas important? Because they show that mainstream Christian doctrine was arrived at by a series of disputes, controversies and compromises, and was not inevitable or obvious. They also show that real Christian theology is a far cry from the simplistic formulations of many modern evangelical churches, which are usually based on penal substitution theology (a combination of the doctrines of total depravity and vicarious atonement).