When Theodosius ascended to the imperial throne in 380, he began on a campaign to bring the Eastern Church back to Nicene Christianity. Theodosius wanted to further unify the entire empire behind the orthodox position and decided to convene a church council to resolve matters of faith and discipline.
Gregory Nazianzus was of similar mind, wishing to unify Christianity. In the spring of 381 they convened the Second Ecumenical Council in Constantinople.
Council of Nicaea in 325 had not ended the
Arian controversy which it had been called to clarify.
Arius and his sympathizers, e.g.
Eusebius of Nicomedia were admitted back into the church after ostensibly accepting the Nicene creed.
Athanasius, bishop of Alexandria, the most vocal opponent of Arianism, was ultimately exiled through the machinations of Eusebius of Nicomedia. After the death of
Constantine I in 337 and the accession of his Arian-leaning son
Constantius II, open discussion of replacing Nicene creed itself began. Up until about 360, theological debates mainly dealt with the divinity of the Son, the second person of the
Trinity. However, because the Council of Nicaea had not clarified the divinity of the
Holy Spirit, the third person of the Trinity, it became a topic of debate. The
Macedonians denied the divinity of the Holy Spirit. This was also known as Pneumatomachianism.
Nicene Christianity also had its defenders: apart from Athanasius, the
Cappadocian Fathers' Trinitarian discourse was influential in the council at
Apollinaris of Laodicea, another pro-Nicene theologian, proved controversial. Possibly in an over-reaction to Arianism and its teaching that Christ was not God, he taught that Christ consisted of a human body and a divine mind, rejecting Christ having a human mind.
 He was charged with confounding the persons of the
Godhead, and with giving in to the heretical ways of
Basil of Caesarea accused him of abandoning the literal sense of the scripture, and taking up wholly with the allegorical sense. His views were condemned in a
Athanasius of Alexandria, in 362, and later subdivided into several different heresies, the main ones of which were the
Polemians and the
Theodosius' strong commitment to Nicene Christianity involved a calculated risk because Constantinople, the imperial capital of the Eastern Empire, was solidly Arian. To complicate matters, the two leading factions of Nicene Christianity in the East, the Alexandrians and the supporters of Meletius in Antioch, were "bitterly divided ... almost to the point of complete animosity".
The bishops of Alexandria and Rome had worked over a number of years to keep the see of Constantinople from stabilizing. Thus, when Gregory was selected as a candidate for the bishopric of Constantinople, both Alexandria and Rome opposed him because of his Antiochene background.
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See of Constantinople
The incumbent bishop of Constantinople was Demophilus, a
Homoian Arian. On his accession to the imperial throne, Theodosius offered to confirm Demophilus as bishop of the imperial city on the condition of accepting the
Nicene Creed; however, Demophilus refused to abandon his Arian beliefs, and was immediately ordered to give up his churches and leave Constantinople.
 After forty years under the control of Arian bishops, the churches of Constantinople were now restored to those who subscribed to the Nicene Creed; Arians were also ejected from the churches of other cities in the Eastern Roman Empire thus re-establishing Christian
orthodoxy in the East.
There ensued a contest to control the newly recovered see. A group led by
Maximus the Cynic gained the support of Patriarch Peter of Alexandria by playing on his jealousy of the newly created see of Constantinople. They conceived a plan to install a cleric subservient to Peter as bishop of Constantinople so that Alexandria would retain the leadership of the Eastern Churches.
 Many commentators characterize Maximus as having been proud, arrogant and ambitious. However, it is not clear the extent to which Maximus sought this position due to his own ambition or if he was merely a pawn in the power struggle. In any event, the plot was set into motion when, on a night when Gregory was confined by illness, the conspirators burst into the cathedral and commenced the consecration of Maximus as bishop of Constantinople. They had seated Maximus on the
archiepiscopal throne and had just begun shearing away his long curls when the day dawned. The news of what was transpiring quickly spread and everybody rushed to the church. The
magistrates appeared with their officers; Maximus and his consecrators were driven from the cathedral, and ultimately completed the tonsure in the tenement of a flute-player.
The news of the brazen attempt to usurp the episcopal throne aroused the anger of the local populace among whom Gregory was popular. Maximus withdrew to
Thessalonica to lay his cause before the emperor but met with a cold reception there. Theodosius committed the matter to
Ascholius, the much respected
bishop of Thessalonica, charging him to seek the counsel of
Pope Damasus I.
Damasus' response repudiated Maximus summarily and advised Theodosius to summon a Council of Bishops for the purpose of settling various Church issues such as the schism in Antioch and the consecration of a proper bishop for the see of Constantinople.
 Damasus condemned the translation of bishops from one see to another and urged Theodosius to "take care that a bishop who is above reproach is chosen for that see."