Which ecumenical council in 381 refined the Nicene Creed and is recognized by the Byzantine Church?

The First Council of Constantinople, held in 381, refined the Nicene Creed and became the second ecumenical council recognized by the Byzantine Church. It was convened during the reign of Emperor Theodosius I, who sought to settle major theological disputes within the Roman Empire.

The council expanded the creed first issued at Nicaea in 325, especially its teaching about the Holy Spirit. The resulting Niceno-Constantinopolitan Creed affirmed that the Holy Spirit proceeds from the Father and is worshiped and glorified with the Father and the Son. It also reinforced the doctrine of the Trinity and rejected several theological movements considered heretical by the council.

The gathering dealt with disputes involving Arianism and related positions that questioned the full divinity of Christ or the Holy Spirit. It also ranked Constantinople after Rome in ecclesiastical precedence because it was the imperial capital. The council should not be confused with the Council of Nicaea, which produced the original creed in 325, or the Council of Chalcedon, which addressed Christological controversies in 451.