Why was the First Council of Constantinople called in 381?

The First Council of Constantinople was summoned by Theodosius I in 381 primarily to establish religious unity within the empire and to clarify the formula of the Nicene Creed.1

Following his 380 edict, which mandated that all subjects adhere to the belief in the consubstantiality of the Father, Son, and Holy Spirit, Theodosius sought to provide ecclesiastical sanction for this doctrine. The council was convened to resolve the sharp theological antagonisms between followers of the Nicene Creed and other Christian groups, specifically by suppressing Arianism and affirming the Trinitarian doctrine of the Holy Spirit's equality with the Father and the Son.231

By formalizing these beliefs, the council solidified the Nicene Creed as the universal norm for Christian orthodoxy, thereby reinforcing the emperor's authority in matters of faith across the Eastern empire.12