council · 381 · Constantinople
Theodosius I gathered about 150 eastern bishops at Constantinople in 381. They completed the Nicene confession with the clauses on the Holy Spirit — the creed most churches still recite — condemned Apollinarianism and the Pneumatomachi, and in canon 3 gave New Rome the primacy of honour after Old Rome, the seed of a long rivalry.
The First Council of Constantinople 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, except for the Western Church, 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 for Chalcedonian Christianity and the Second Council of Ephesus for the Oriental Orthodox Churches.
The Arian aftermath and the deity of the Holy Spirit; Apollinaris' truncated Christ.
The creed completed with the Spirit's clauses — worshipped and glorified with Father and Son; Apollinarianism condemned; canon 3 gives New Rome the primacy of honour after Rome.
Wikipedia · Catholic Encyclopedia — First Council of Constantinople · Fordham sourcebook — documents of Constantinople I · OrthodoxWiki — Second Ecumenical Council · The Seven Ecumenical Councils — Constantinople I (NPNF II/14)