The Council of Nicea had not ended the controversy in regard to the identity of Christ. Arius' view that Jesus had a beginning and was created had taken over the Eastern part of the Roman Empire. After Constantine died and his son, Constantius II, helped spread Arius' view and banned Athaneous's view, opening discussions of changing the Nicene Creed had begun.
Up until this point, the Nicene Creed was a duality. The Council of Nicea (which was deemed heresy) had claimed that Jesus IS God, but did not have a position on a third part of the trinity. So up until this point, there was the Father and the Son. The spirit was the spirit of the Father and Son, but not a separate entity.
In 380 AD, Theodosius ascended to the throne and began a campaign to bring the Eastern church back to the Nicene Creed. Demophilus was a bishop in Constantinople and a believer of Arius' view. Theodosius offered to make Demophilus a bishop of the imperial city under one condition; he accepts the Nicene Creed. Demophilus did not believe in selling out his faith for power so Theodosius forced him to give up his churches and leave Constantinople. After 40 years of being under the control of bishops sharing Arius's view, the new bishops installed by Theodosius restored the Nicene creed back into the church. Theodosius wanted to have the entire empire unified under a single orthodox position and formed the council of Constantinople to resolve all matters of faith and discipline.
Anyone who believed in Arius's views was denied admission to the council unless they accepted the Nicene Creed. A total of 36 of 150 bishops were not admitted for failure to declare Nicene Creed as truth.
The council then proceeded in creating the "Four Canons" of the church.
Canon 1 is condemnation of all shades of Arius's views.
Canon 2 renews the Nicene legislation imposed on bishops.
Canon 3 is that the Bishop of Constantinople shall have the honor as bishop of all Rome.
Canon 4 declared that Maximus (a believer of Arius's view) was not a bishop nor are any who were ordained by him in any ranks.
With these canons of the church in place, anyone who believed in Arius's view, or appointed by anyone who believed, or currently held a position - no longer had any authority in the church by the order of the Roman Empire and the Council of Constantinople.
The third canon was the most concerning for people as it demoted all the patriarchs of Antioch and Alexandria who taught Arius's views. This put them under the direct ruling of the Bishop of Constantinople, a position that could only be held by someone who agreed to the Nicene Creed. Because of this third Canon, the supremacy of the lead bishop (the Pope) was now the supreme authority in the church.
There was some protest to this in regards to demoting Antioch's authority which was appointed by Christ and not by a gathering of one-sided bishops, but ultimately majority ruled that they would respect Antioch and Alexandria as being older churches while the bishop of Constantinople would still be the supreme authority.
The council also wanted to outline and clarify points within the creed to make it so no one could question the meaning and it would be an all-inclusive statement of faith that gave the Catholic church full authority, added baptism, and details of Christ's death.
There was one final matter to discuss which was the Holy Spirit. This became a topic of debate as well since there was a group of people who said the Holy Spirit was of the Father but not a divine being in itself. They claimed that the idea was pagan from Neoplatonism (Mind, Soul, Matter - all under "One Good") ideology. They claimed that this pagan idea made the Father the mind, Christ the Matter, and Holy Spirit is the Soul, all making up "One Goodness" and adopting a trinitarian ideology similar to that of Egyptian Triune gods 'Anu, Enlil, and Ea', Babylonian's 3 headed god, and India's triune god 'Brahma, Vishnu, Shiva'.
The council disagreed and said that the Holy Spirit should be a part of God because it is a separate thing and all things in this universe are things of three so God also must be a three in one. Thus the Nicene Creed was amended (additions in red):
We 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 only-begotten son of God, begotten of the Father before all worlds. Light of Light, very God of very God, begotten not made, consubstantial with the Father (same substance), by whom all things were made. Who for us men, and for our salvation, came down from heaven and was incarnate by the Holy Ghost and of the Virgin Mary, and was made man. 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 on the right hand of the Father from which he shall come again with glory to judge the quick and the dead whos kingdom shall have no end. And in the Holy Ghost, the lord and giver of life who proceeds from the Father who with the Father and the Son together is worshipped and glorified who spoke by the prophets. In one holy catholic and apostolic church; we acknowledge one baptism for the remissions of sins; we look for the resurrection of the dead, and the life of the world to come. Amen.