From The Art and Popular Culture Encyclopedia
In Christianity, Trinity is the doctrine that God is one being who exists, simultaneously and eternally, as a mutual indwelling of three persons (not to be confused by "person"): the Father, the Son (incarnate as Jesus of Nazareth), and the Holy Spirit. Since the 4th century, in both Eastern and Western Christianity, this doctrine has been stated as "three persons in one God," all three of whom, as distinct and co-eternal persons, are of one indivisible Divine essence, a simple being. The doctrine also teaches that the Son Himself has two distinct natures, one fully divine and the other fully human. Supporting the doctrine of the Trinity is known as Trinitarianism. The majority of Christians are Trinitarian, and regard belief in the Trinity as a test of Christian orthodoxy.
Opposing nontrinitarian positions held by some groups include Binitarianism (two deities/persons/aspects), Unitarianism (one deity/person/aspect), the Godhead (Latter Day Saints) (three separate beings, one in purpose) and Modalism (Oneness).
Historically, the doctrine of Trinitarianism is of particular importance. The conflict with Arianism, as well as other competing theological concepts during the fourth century, became the first major doctrinal confrontation in Church history. It had a particularly lasting effect within the Western Roman Empire where the Germanic Arians and Nicene Christians formed a segregated social order.