First Council of Constantinople

The First Council of Constantinople (Ancient Greek: Κωνσταντινούπολις Konstantinoúpolis) was a council of Christian bishops convened in Constantinople in AD 381 by the Roman Emperor Theodosius I. This second ecumenical council, an effort to attain consensus in the church through an assembly representing all of Christendom, confirmed the Nicene Creed, expanding the doctrine thereof to produce the Niceno–Constantinopolitan Creed, and dealt with sundry other matters. It met from May to July 381 in the Church of Hagia Irene and was affirmed as ecumenical in 451 at the Council of Chalcedon.

When Theodosius ascended to the imperial throne in 380, he began on a campaign to bring the Eastern Church back to Nicene Christianity. Theodosius wanted to further unify the entire empire behind the orthodox position and decided to convene a church council to resolve matters of faith and discipline.Gregory Nazianzus was of similar mind, wishing to unify Christianity. In the spring of 381 they convened the Second Ecumenical Council in Constantinople.

The Council of Nicaea in 325 had not ended the Arian controversy which it had been called to clarify. Arius and his sympathizers, e.g. Eusebius of Nicomedia were admitted back into the church after ostensibly accepting the Nicene creed. Athanasius, bishop of Alexandria, the most vocal opponent of Arianism, was ultimately exiled through the machinations of Eusebius of Nicomedia. After the death of Constantine I in 337 and the accession of his Arian-leaning son Constantius II, open discussion of replacing Nicene creed itself began. Up until about 360, theological debates mainly dealt with the divinity of the Son, the second person of the Trinity. However, because the Council of Nicaea had not clarified the divinity of the Holy Spirit, the third person of the Trinity, it became a topic of debate. The Macedonians denied the divinity of the Holy Spirit. This was also known as Pneumatomachianism.