First Council of Constantinople, (381), the second ecumenical council of the Christian church, summoned by the emperor Theodosius I and meeting in Constantinople. Doctrinally, it adopted what became known to the church as the Niceno-Constantinopolitan Creed (commonly referred to as the Nicene Creed), which effectively affirmed and developed the creed earlier promulgated at the First Council of Nicaea in 325 (Creed of Nicaea). The Nicene Creed was, however, probably not an intentional enlargement of the Creed of Nicaea but rather an independent document based on a baptismal creed already in existence. The Council of Constantinople also declared finally the Trinitarian doctrine of the equality of the Holy Spirit with the Father and the Son. Among the council’s canons was one giving the bishop of Constantinople precedence of honor over all other bishops except the bishop of Rome, “because Constantinople is the New Rome.”

Though only Eastern bishops had been summoned (about 150 in all), the Greeks claimed this council to be ecumenical. Pope Damasus I in Rome appears to have accepted the creed but not the canons, at least not the canon upon the precedence of Constantinople. (Rome indeed accepted the precedence of Constantinople, next to Rome, only during the life of the Latin Empire of Constantinople, created in the 13th century during the Fourth Crusade.) In both East and West, nevertheless, the council came to be regarded as ecumenical.