First Council of Constantinople
The First Council of Constantinople was called by Byzantine Emperor Theodosius I in 381 to confirm the Nicene Creed and deal with other matters of the Arian controversy. This was the second of the first seven ecumenical councils. This council, which was attended by 186 bishops, settled two basic issues: the deity of the Holy Spirit and the true humanity of Christ.
The council affirmed the original Nicene Creed of faith as far as it went but expanded the discussion on the Holy Spirit to combat heresies. It is therefore called the "Nicene Creed of 381" or the Niceno-Constantinopolitan Creed to distinguish it from that of the First Council of Nicaea (325 A.D.).
Specifically, this council expanded the 3rd article of the creed dealing with the Holy Spirit to say that he is "the Lord, the Giver of Life, Who proceeds from the Father, With the Father and the Son he is worshipped and glorified".
The Council's decision on the Holy Spirit gave official endorsement to the concept of the Trinity. By the end of the 4th century, the Byzantine Emperor Theodosius "issued a decree that the doctrine of the Trinity was to be the offical state religion and that all subjects shall adhere to it". (Cf Rome's Christian Emperors, to 410 CE, Antiquity Online)