Trinity refers to the teaching that the one God comprises three distinct, eternally co-existing persons; the Father, the Son (incarnate in Jesus Christ), and the Holy Spirit. Together, these three persons are sometimes called the Godhead, although there is no single term in use in Scripture to denote the unified Godhead. In the words of the Athanasian Creed, an early statement of Christian belief, "the Father is God, the Son is God, and the Holy Spirit is God, and yet there are not three Gods but one God". They are distinct from another: the Father has no source, the Son is begotten of the Father, and the Spirit proceeds from the Father. Though distinct, the three persons cannot be divided from one another in being or in operation.The Trinity is an essential doctrine of mainstream Christianity. "Father, Son and Holy Spirit" represents both the immanence and transcendence of God. God is believed to be infinite and God's presence may be perceived through the actions of Jesus Christ and the Holy Spirit. According to this doctrine, God is not divided in the sense that each person has a third of the whole; rather, each person is considered to be fully God (see Perichoresis). The distinction lies in their relations, the Father being unbegotten; the Son being begotten of the Father; and the Holy Spirit proceeding from the Father and (in Western Christian theology) from the Son. Regardless of this apparent difference, the three 'persons' are each eternal and omnipotent.The word trias, from which trinity is derived, is first seen in the works of Theophilus of Antioch. He wrote of "the Trinity of God (the Father), His Word (the Son) and His Wisdom (Holy Spirit)". The term may have been in use before this time. Afterwards it appears in Tertullian.In the following century the word was in general use. It is found in many passages of Origen.
Trinitarianism denotes those Christians who believe in the concept of the Trinity. Almost all Christian denominations and Churches hold Trinitarian beliefs. Although the words "Trinity" and "Triune" do not appear in the Bible, theologians beginning in the 3rd century developed the term and concept to facilitate comprehension of the New Testament teachings of God as Father, God as Jesus the Son, and God as the Holy Spirit. Since that time, Christian theologians have been careful to emphasize that Trinity does not imply three gods, nor that each member of the Trinity is one-third of an infinite God; Trinity is defined as one God in three Persons.
Nontrinitarianism refers to theology that rejects the doctrine of the Trinity. Various nontrinitarian views, such as adoptionism or modalism, existed in early Christianity, leading to the disputes about Christology. Nontrinitarianism later appeared again in the Gnosticism of the Cathars in the 11th through 13th centuries, and by groups with Unitarian theology in the Protestant Reformation of the 16th century, and in the Age of Enlightenment of the 18th century, and in some groups arising during the Second Great Awakening of the 19th century.