The Christian doctrine of the Trinity teaches the unity of Father, Son, and Holy Spirit as three persons in one Godhead. The doctrine states that God is the Triune God, existing as three persons, or in the Greek hypostases, but one being. (Personhood in the Trinity does not match the common Western understanding of "person" as used in the English language—it does not imply an "individual, self-actualized center of free will and conscious activity." To the ancients, personhood "was in some sense individual, but always in community as well." Each person is understood as having the one identical essence or nature, not merely similar natures.) Since the beginning of the third century the doctrine of the Trinity has been stated as "the one God exists in three Persons and one substance, Father, Son, and Holy Spirit." Trinitarianism, belief in the Trinity, is a mark of Roman Catholicism, Eastern and Oriental Orthodoxy as well as of the "mainstream traditions" arising from the Protestant Reformation, such as Anglicanism, Baptist, Methodism, Lutheranism and Presbyterianism. The Oxford Dictionary of the Christian Church describes the Trinity as "the central dogma of Christian theology".

The concept of Trinitarianism came about 300 years after Jesus' passing, when certain non-standard views such as Arianism were coming to prominence (cf. First Council of Nicaea). A central concern among Church leaders was that the holiness of Jesus be regarded and reinforced in teaching, such that Jesus would not be viewed as a mere prophet, but as the Son of God (cf. Arian controversy). It was also believed that God the Father employed the image of Jesus as an angel of divine manifestation, such that Jesus —as known in iconic form —had become the human face and personae of the immaterial God (cf. illumination). Leaders were also concerned that due to the invisibility of God and the iconic visibility of Jesus, that Jesus might increasingly be viewed as the deity, rather than as the Son and messiah (cf. Personhood of the Holy Spirit). Trinitarianism thus incorporated God the Father, Jesus the Son, and the Holy Spirit (a manifestation of God's presence) into a single concept such that made clear that God the Father is at the head of the Trinity and Jesus the Son was the human being in whom God's presence was most manifest. The Trinity thus expanded upon Jesus' statements regarding God's presence within him, and his place within God (for example, 1 John 14:09-10 and 1 John 4:15-16).

Trinitarianism contrasts with Nontrinitarian positions which include Binitarianism (one deity/two persons), Unitarianism (one deity/one person), the Oneness belief held by certain Pentecostal groups, Modalism, and The Church of Jesus Christ of Latter-day Saints' view of the Godhead as three separate beings who are one in purpose rather than essence.

The New Testament does not have an explicit doctrine of the Trinity. However, Southern Baptist theologian Frank Stagg emphasizes that the New Testament does repeatedly speak of the Father, the Son, and the Holy Spirit to "compel a trinitarian understanding of God." The doctrine developed from the biblical language used in New Testament passages such as the baptismal formula in Matthew 28:19 and took substantially its present form by the end of the 4th century as a result of controversies in which some theologians, when speaking of God, used terms such as "person", "nature", "essence", "substance", terms that had never been used by the Apostolic Fathers, in a way that the Church authorities considered to be erroneous.

Some deny that the doctrine that developed in the fourth century was based on Christian ideas, and hold instead that it was a deviation from Early Christian teaching on the nature of God or even that it was borrowed from a pre-Christian conception of a divine trinity held by Plato.