Since we decided in Bible Study a few weeks ago that the opposite of Atheism is Unitarianism ("I believe in nothing" vs. "I believe in everything (except Orthodox Christian teachings)" -- and yes, I know that is a drastic oversimplification), I was reminded of this week's heresy.
Socinianism is a 15th century protestant antitrinitarian heresy that has been embraced by the followers of Unitarianism. Socinianism rejects orthodox teachings of God’s knowledge, the Trinity, the divinity of Christ (elements of Arianism, Ebionitism and Psilanthropism) and salvation. Christ was believed to have not existed before the virginal birth of Jesus (moderated Adoptionism); some Socinianites believed that Joseph was the father of Jesus and rejected the virginal birth. They reject the doctrine of original sin and teach that humans were created mortal and would have died naturally whether or not Adam and Eve ate from the Tree of Good and Evil. Because there is no original sin, they do not believe in the need for atonement. They teach that God’s omniscience is limited to necessary truths (what will definitely happen in the future) and not to contingent truth (what only might happen in the future); they felt if God knew everything that would happen, free will could not exist.