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.

Background
The Council of Nicaea did not end the Arian controversy which it had been called to clarify. By 327 Emperor Constantine had begun to regret the decisions that had been made at the Nicene Council. He granted amnesty to the Arian leaders and exiled Athanasius because of Eusebius of Nicomedia. Even during numerous exiles, Athanasius continued to be a vigorous defender of Nicene Christianity against Arianism. The Cappadocian Fathers also took up the torch and their Trinitarian discourse was influential in the council at Constantinople.

Up until about 360, theological debates dealt mainly with the Divinity of Jesus, the 2nd person of the Trinity. However, because the First Council of Nicaea had not clarified the divinity of the Holy Spirit, it became a target for heretics. The Macedonians denied the divinity of the Holy Spirit. This was also known as Pneumatomachianism.

The council affirmed the original Nicene Creed of faith as true and an accurate explanation of Scripture. This council also developed a statement of faith which included the language of Nicea, but expanded the discussion on the Holy Spirit to combat heresies. It is therefore called the "Nicene Creed of 381" and was a commentary on the original Nicene formula.

It expanded the 3rd article of the creed dealing with the Holy Spirit, as well as some other changes. Regarding the Holy Spirit, the article of faith said 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 statement of "proceeding from the Father" is seen as significant because it established that the Holy Spirit must be of the same essence (ousia) as God the Father.

This Council's decision regarding the Holy Spirit also 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". (See Rome's Christian Emperors, to 410 CE, Antiquity Online)

the Wikipedia article First Council of Constantinople.