As per Randy Rogers Law:
After the First Ecumenical Council that took place in AD 325 to establish the foundations of the Nicene Creed and bring a unified doctrine to Christianity, the Roman empire was struck with turbulence regarding the official state religion. Despite bishops from across the empire gathering and agreeing upon a single doctrine regarding the Father and the Son, the First Council of Nicaea never addressed the third element of the Trinity: the Holy Spirit. Along with this newly-rising debate that threatened the stability of Nicene Christianity, believers belonging to other factions attempted to assert their power by influencing those within power. Arianism, Meletism and paganism all attempted to revive their standing before Emperor Theodosius I campaigned to establish Nicene Christianity as the official state religion in AD 380. With the 55-year struggle at an end, Theodosius called to convene the First Council of Constantinople, the second Ecumenical Council in the history of Christianity, to reconfirm the Nicene Creed that was established in the First Council of Nicaea.
While attempts to overturn Nicene Christianity had failed, for the most part, the events leading up to the First Council of Constantinople were tumultuous at best. Even despite recognizing Nicene Christianity as the state religion, many high-ranking officials within Constantinople were still of Arian belief. Gathering supporters from Alexandria and Antioch together was also considered a gamble, as Nicene and Meletian supporters were at odds with each other. Spurred on as well by a conspiracy to seat a bishop that would allow Alexandria to maintain control of the Eastern Churches, the conspiracy was discovered, and the issue ultimately brought before Pope Damascus who issued a decree to the Emperor to summon a council to settle the matter and instate a bishop worthy of the position.
In the proceedings that followed, the consecration of Maximus, the bishop that conspired to seize the open position in Constantinople, was declared invalid and Gregory Nazianzus was instated by Theodosius. Despite this installation, Gregory surprisingly offered his resignation when he lost the support and the confidence of the fellow bishops. When his resignation was granted, an unbaptized civil official named Nectarius was set to succeed him, and power over the churches transferred in Constantinople despite the efforts of Maximus and Alexandria, though this would be corrected later by Pope Damascus.
Apart from this matter, the Niceno-Constantinopolitan Creed was also instated. A more structurally sound revision to the original Nicene Creed, though it clarifies confusion regarding the Holy Spirit as a definitive part of the Trinity. It also addresses separate matters regarding the Church itself, baptism and resurrection of the dead. This particular portion of the First Council of Constantinople has come under scrutiny, however, and many historians believe that the Niceno-Constantinopolitan Creed was simply a decree from the bishops within the empire and not necessarily a part of the official delegation of the council. Despite this debate, it further solidified the position of Nicene Christianity within the Roman empire.
Many minor canons were also discussed by the gathering, though much debate surrounded the Third Canon that prioritized power of the Eastern churches in Constantinople and made the bishop there second only to the Bishop of Rome. Pope Damascus would address this the following year (AD 382) and historians assert that, while this council was regarded as ecumenical, the Pope is likely to have only approved of the revision of the Nicene Creed. Some believe, however, that the aftermath of this council is evidence that power between the Western and Eastern portions of the churches was reaching a breaking point, and the West was beginning to lose its influence as Constantinople began to establish itself as a great seat of power within Christianity.