One God, and an Arithmetic That Is Not Arithmetic

The doctrine holds that God is one being (Greek ousia) in three persons (hypostases), Father, Son and Holy Spirit, each of them fully and equally God, so that they are not three gods and not three masks of one actor. It is the teaching that most sharply distinguishes Christianity from the other two great monotheisms, both of which read it as a compromise of the oneness of God.

The word "Trinity" is in no book of the Bible. What the New Testament supplies is the raw material: the three named together in the baptismal command of Matthew 28:19 and the blessing of 2 Corinthians 13:14, alongside a firm inheritance of Jewish monotheism and the plain fact that the early church prayed to Jesus and to the Spirit as to God.

The doctrine is the second-to-fourth-century synthesis that reconciled those data. Tertullian, writing in Latin around the year 200, seems to have coined trinitas and the formula "one substance, three persons." The point of the later definitions was never to explain the mystery but to fence it, to rule out the readings the church judged to have gone wrong.