The Trinity is one God: Father, Son, and Holy Spirit (BCP, p. 852). The term is from the Latin tri, "three," and unitas, "unity." The term was devised by Tertullian to express the mystery of the unity-in-diversity of God. Trinity means "threefold unity." The corresponding word in Greek is ho trias, which means "the Triad." The Trinity is a perfect relationship of love in which neither unity nor distinctness of the divine persons is compromised. God's life is understood to be dynamic, loving, and available to be shared in relationship with humanity for salvation. The term "economic Trinity" has been applied to the life of the Trinity in time and space, in the "economy" of salvation; as distinguished from the "immanent Trinity" which refers to the inner life of God beyond the limits of time and space. It may be said that our experience of the economic Trinity leads us to know the immanent Trinity and that God's self-revelation corresponds to God's essential nature. However, the helpfulness of this distinction should not be overemphasized because there is only one divine trinitarian life. Karl Rahner states, "The economic Trinity is the immanent Trinity, and vice versa."
Christian theology is the heir of both uncompromising biblical monotheism and the Platonic, Aristotelian, and Stoic emphases on the unity and simplicity of God. However, the NT ascribed a place of equality with God to the Word of God who became incarnate in Jesus of Nazareth (e.g., Jn 1:1-18, Col 1:15-20). The Spirit of God was also included in the divine life (1 Cor 2:10-13). The church took several centuries to work out a reasonably acceptable way to express the complex relation of Father, Son, and Spirit. The nearly complete doctrine of the Trinity announced at Constantinople in 381 held that God is one Being (ousia) in three equal and consubstantial persons or hypostases: the Father uncreated, the Son uncreated but begotten, the Spirit proceeding from the Father (and, in the western version of the Creed, the Son). The Athanasian Creed states that "we worship one God in Trinity, and Trinity in Unity, neither confounding the Persons, nor dividing the Substance" (BCP, p. 864). Article I of the Articles of Religion affirms that in the unity of God "there be three Persons, of one substance, power, and eternity; the "Father, the Son and the Holy Ghost" (BCP, p. 867). See Filioque; see Homoousios; see Perichoresis; see Trinity Sunday.