Christianity is a monotheistic religion, meaning that it teaches the existence of one God (specifically, the God of the Jews). It shares this belief with two other major world religions, Judaism and Islam. However, Christian monotheism is a unique kind of monotheism. It holds that God is One, but that three distinct "persons" constitute the one God: the Father, the Son (Christ), and the Holy Spirit. This unique threefold God of Christian belief is referred to as the Trinity (tri + unity).
The doctrine of the Trinity is accepted by all "mainstream" branches of Christianity (Catholic, Eastern Orthodox, Protestant and Anglican) and in fact is one of the major defining factors for what is considered mainstream. But the doctrine of the Trinity is rejected by a number of other faiths that consider themselves Christian, including Mormonism, Jehovah's Witnesses, and Unitarian Universalism.
History of Belief in the Trinity
Foundations of the concept of the Trinity can be seen in the New Testament[#1] and in the teachings of Christian writers as early as the end of the first century[#2], but the clearest expression of the concept came with Tertullian, a Latin theologian who wrote in the early third century. Tertullian coined the words "Trinity" and "person" and explained that the Father, Son and Holy Spirit were "one in essence - not one in Person"[#3].
About a century later, in 325, the Council of Nicea set out to officially define the relationship of the Son to the Father, in response to the controversial teachings of Arius. Led by bishop Athanasius, the council established the doctrine of the Trinity as orthodoxy and condemned Arius' teaching that Christ was the first creation of God. The creed adopted by the council described Christ as "God of God, Light of Light, very God of very God, begotten, not made, being of one substance with the Father."[#4]
Of course, this was not the end of the controversy. Debate over how the creed (especially the phrase "one substance") ought to be interpreted continued to rage for decades. But for the most part, the issue of the Trinity was settled at Nicea and never again became a focus of serious controversy. Later debates would center on issues related to Christology and the doctrine of salvation.
Since Nicea, theological discussion of the Trinity has primarily consisted of attempts to understand and explain such a unique concept. Gregory of Nyssa, in his treatise, That There are Not Three Gods, compared the divinity shared by the three persons of the Trinity to the common "humanness," or human nature, that is shared by individual human beings. (Ironically, this initally promising explanation has been seen by some to yield a conclusion quite opposite than the title of his work.)
Saint Augustine, one of the greatest thinkers of the early church, described the Trinity as comparable to the three parts of an individual human being: mind, spirit, and will. They are three distinct aspects, yet they are inseparable and together constitute one unified human being.
Modern Denominational Statements on the Trinity
The mystery of the Most Holy Trinity is the central mystery of the Christian faith and of Christian life. -- Roman Catholicism
The fundamental truth of the Orthodox Church is the faith revealed in the True God: the Holy Trinity of the Father, the Son, and the Holy Ghost. -- Greek Orthodox Archdiocese of America
We teach that the one true God. is the Father and the Son and the Holy Ghost, three distinct persons, but of one and the same divine essence, equal in power, equal in eternity, equal in majesty, because each person possesses the one divine essence. -- Lutheran Church (Missouri Synod)
We trust in the one triune God. -- Presbyterian Church (USA)
The eternal triune God reveals Himself to us as Father, Son, and Holy Spirit, with distinct personal attributes, but without division of nature, essence, or being. -- Southern Baptist Convention
There is but one living and true God, everlasting, without body or parts, of infinite power, wisdom, and goodness; the maker and preserver of all things, both visible and invisible. And in unity of this Godhead there are three persons, of one substance, power, and eternity-the Father, the Son, and the Holy Ghost. -- United Methodist Church
[#1]: Matthew 28:19; John 1:1; John 10:30.
[#2]: Epistle of Ignatius to the Ephesians (ANF 1.58); The Martyrdom of Polycarp 14 (ANF 1.42).
[#3]: ANF 3.621; c. 213 CE
[#4]: William Placher, Readings in the History of Christian Theology, 53.