The doctrine that God is one being existing as three co-equal, co-eternal, consubstantial persons: Father, Son, and Holy Spirit. One God in three persons; three persons in one God. Neither three gods (tritheism) nor one God in three modes (modalism). The Nicene Creed (325 AD) establishes the consubstantiality of Father and Son. The Council of Constantinople (381 AD) completes the formulation by affirming the full divinity of the Holy Spirit. The Cappadocian Fathers (Basil of Caesarea, Gregory of Nyssa, Gregory of Nazianzus) provide the definitive theological vocabulary: one ousia (substance/being), three hypostases (persons/subsistences). The Western theological tradition (Augustine) develops the psychological analogy for the Trinity: memory/understanding/will as an image of Father/Son/Spirit within the human mind. Trinitarianism is rejected by Unitarians, Jehovah’s Witnesses, and certain Oneness Pentecostals.