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(1) In a broad sense, a synonym for the anti-Trinitarians.
(2) In a narrower sense, the Protestant anti-Trinitarians, who since the Reformation of the 16th century constitute the left, rationalist wing of Protestantism. The term “Unitarian” became current in the mid-16th to early 17th centuries and from 1638 was adopted by the adherents of Unitarianism themselves.
Besides denying the dogma of the Trinity, which they view as a recidivist pagan polytheism, the Unitarians reject the Christian dogma about the Fall and the sacraments, including those recognized by the Protestants, such as baptism and communion. The Unitarians were continually persecuted by both the Catholics and the Orthodox. In the late 16th and early 17th centuries, the Unitarians were based in Poland (the Socinians are a branch of Unitarians) and Hungary, and from the late 17th century, in England. (A law providing for the death penalty for Unitarians in Great Britain was revoked only in 1813.) In the early 19th century the Unitarians achieved their greatest numbers in the USA, with Harvard University being the most important center. As of the 1970’s, most Unitarians are in the USA (about 150,000) and Great Britain (about 20,000).