Nicaea, Constantinople, and a Single Letter

The doctrine was forced into precision by a fourth-century crisis. Arius, a presbyter of Alexandria, taught that the Son was the first and highest of God's creatures, "there was when he was not", so that the Son was not eternal and not God in the full sense. The First Council of Nicaea in 325 answered with a word borrowed from philosophy: the Son is homoousios, "of the same being," as the Father. It differs from the rival homoiousios, "of like being," by a single Greek letter, the iota, the origin of the phrase about caring for an iota.

Nicaea did not end the argument; it began sixty years of it. Athanasius of Alexandria defended the word through repeated exiles, and the three Cappadocian Fathers, Basil of Caesarea and the two Gregorys, supplied the settled grammar of one ousia in three hypostases. The First Council of Constantinople in 381 affirmed the full divinity of the Holy Spirit and issued the creed still recited in the liturgies today. In the West, Augustine's On the Trinity added the influential "psychological" analogy of memory, understanding and will within a single mind.