Why was the Creed written?
By the fourth century, a number of false teachings about Jesus Christ, the Holy Spirit, and the Church had developed. In order to combat these heresies and clearly state the Truth, two Ecumenical Councils were held.
The Council of Nicea, held in the year 325 A.D., clarified the Orthodox doctrine of Jesus Christ by producing the first portion of the Creed:
I believe in one God, the Father almighty, maker of heaven and earth, and of all things visible and invisible. And in one Lord Jesus Christ, the Son of God, the only-begotten, begotten of the Father before all ages. Light of Light; true God of true God; begotten, not made; of one essence with the Father, by whom all things were made; who for us men and for our salvation came down from heaven, and was incarnate of the Holy Spirit and the Virgin Mary, and became a man. And He was crucified for us under Pontius Pilate, and suffered, and was buried. And the third day He rose again, according to the Scriptures, and ascended into heaven, and sits at the right hand of the Father; and He shall come again with glory to judge the living and the dead; whose Kingdom shall have no end.
In A.D. 381, the Council of Constantinople defined the correct belief concerning the Holy Spirit, thereby editing the second portion of the Creed.
And in the Holy Spirit, the Lord, the Giver of Life, who proceeds from the Father; who with the Father and the Son together is worshiped and glorified; who spoke by the prophets. In one Holy, Catholic, and Apostolic Church. I acknowledge one baptism for the remission of sins. I look for the resurrection of the dead, and the life of the world to come. Amen.
The Nicene-Constantinopolitan Creed has been professed by Orthodox Christians in its entirety, without alteration, since the fourth century.
Source: OUR FAITH by Father John Matusiak