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Unitarianism:
There are three distinct schools of Unitarian thought:
'Biblical Unitarianism' - God is one being Who consists of one person - the Father. The Messiah is the Son of God, but not God Himself, nor even a pre-existent being. Jesus Christ is the Son of God; a mortal man (who did not exist before his conception and subsequent birth), conceived by the Holy Spirit, who later received immortality and divine nature. Even now he is still not God, but rather an exalted man. Biblical Unitarians are distinguished from Rationalist Unitarians by their strict adherence to Sola Scriptura and their belief that Scripture is both inspired and inerrant. Christadelphians are Biblical Unitarians, as are the majority of historical persons mentioned in this article (such as Francis David.) Other famous BUs include Isaac Newton and John Locke.
'Rationalist Unitarianism' - God is one being Who consists of one person - the Father. Jesus is not the Son of God, but merely a "good and wise man" who taught others how to lead a better life. Rationalist Unitarianism emerged from the German Rationalism of the 19th Century. Its proponents took a highly intellectual approach to religion, rejecting most of the miraculous events in the Bible (including the virgin birth.) They embraced evolutionary concepts, asserted the "inherent goodness of man" and abandoned many principles of Christianity. James Martineau (1805-1900) was one of their most prominent members. Rationalist Unitarianism is distinguished from Deism (with which it nevertheless shares many features) by the fact that RUs believe in a personal deity Who interacts with His creation, while Deists see God as an impersonal force which remains aloof from creation.
'Unitarian Universalism' - There is no formal creed or set of beliefs required to join a Unitarian Universalist (UU) congregation reflecting an institutional consolidation between Unitarianism and Universalism in 1961 in the United States. Today, many Unitarian Universalists no longer consider themselves to be Christians. Of those that do, there is no requirement of unitarian or trinitarian belief other than what the individual concludes on their own.
The development of the various forms of Unitarianism is intermingled. However, Unitarian Universalism is the most recent form, being a product of the 1961 merger while Biblical Unitarianism is the oldest. Modern Unitarians in Europe are primarily Biblical or Rational Unitarians, while Unitarian Universalism is the predominant form of Unitarianism in the United States and Canada.
Universalism:
In comparative religion, a universalist religion is one that holds itself true for all people; it thus allows all to join, regardless of ethnicity. In contrast, ethnic religions, like ethnicity itself, can be determined not just by genealogy, but by geography, language, and other social boundaries. In that sense, Islam and Christianity are universalist religions. Judaism and Hinduism are ethnic religions.
The name Universalism refers to certain religious denominations of universalist religions, which as a core principle adhere to standards and rituals which are convergent rather than divergent, often espousing themselves as alternatives to denominations based on dogmatic or factionalized differences.
In Christian theology, universalism, or universal salvation, is the doctrine that all people will eventually be saved and go to heaven when they die. Some universalists believe that some will endure a limited period of punishment before going to heaven. By doctrine, almost all denominations of Christianity reject universalism as a heresy, although many modern adherents believe in universalism.
