Symbol of the Trinity: three persons, one essence

This series of theological disputes arose in response to late second-century monarchianism, which opposed the Trinitarian concept in the interests of affirming the unity of the Godhead. Dynamic or adoptionist monarchianism held that Jesus was a mere man and that the powers of Christ had been bestowed on him at baptism. Paul of Samosata (third century), bishop of Antioch, propagated this view. Against this, orthodox teachers argued that the Son and the Father were homoousios or of "one substance." Modal monarchianism asserted that God was an undivided unity who expressed himself at different, successive times in three different modes. Sabellius (third century) taught this opinion. Against modal monarchianism, orthodox teachers argued that there were three distinct persons in the Godhead. Against both forms of monarchianism, orthodox teachers held that Christ had been born from eternity.

The threat of heretical teaching with regard to the Trinity was renewed in the fourth century with the appearance of Arianism which taught that the Son was a created being, subordinate to the Father. This teaching was condemned at the Council of Nicea I (325). This council emphasized that the Father and the Son were homoousios "of one substance." This was nevertheless disputed throughout the fourth century, and some Christians were of the Arian persuasion well into the middle ages. Athanasius of Alexandria was the principal opponent of Arianism and instrumental in the victory of Nicene orthodoxy in credal terms, at least, at the Council of Constantinople I (381).

Athanasius thus secured the foundations of the doctrine of the Trinity. He did not, however, speak of three "persons" in the Godhead, preferring rather to think of the Trinity in terms of divine activity. It remained for the Cappadocian Fathers, hearkening back to Origen and Tertullian, to set in place the vocabulary of the orthodox teaching concerning the Trinity. To accomplish this, the Cappadocians complemented the term homoousios or "one substance" with the term hypostasis for each of the three "persons" of the Trinity. Thus Christians in both East and West learned to speak of the Godhead as having one, undivided essence and three persons.