Christians affirm the Holy Trinity: that the one God exists eternally as three persons, the Father, the Son, and the Holy Spirit, who are distinct yet share one divine being. The doctrine was worked out across the first Christian centuries against specific rival positions, most consequentially Arianism, the teaching associated with the presbyter Arius that the Son was a created being, exalted above all else but not eternal or fully divine, summarized in the slogan there was a time when he was not. The Council of Nicaea in 325 condemned that view and declared the Son homoousios, of one substance, with the Father; the fuller doctrine of the Spirit's divinity was affirmed at the Council of Constantinople in 381, giving the creed recited in most churches today its essentially final form.
Much of the technical vocabulary in which the doctrine is now stated, one divine essence (ousia) subsisting in three distinct persons or hypostases, was worked out by the fourth-century Cappadocian Fathers, Basil of Caesarea and the two Gregorys, of Nazianzus and of Nyssa, whose careful distinction between the terms for shared being and individual person gave the later church a stable way to affirm both the unity and the threeness without collapsing into either a single undifferentiated God or three separate gods.