During his difficult tenure at Constantinople, Gregory delivered five orations, later called simply the Theological Orations, defending the full divinity of the Son and, more originally than most defenders of Nicaea had yet managed, the full divinity of the Holy Spirit, working out language for how three persons could be spoken of as one God without collapsing into either three gods or a single person wearing three masks.
The orations were influential enough, and Gregory's oratory skilled enough, that they earned him within his own lifetime the title ho Theologos, the Theologian, a title Eastern Christian tradition has otherwise given only to John the Evangelist and, later, Symeon the New Theologian. In 381 the emperor Theodosius I, having restored Nicene Christianity as the state religion, installed Gregory as Archbishop of Constantinople and had him preside over the opening of the Second Ecumenical Council, called specifically to confirm the Trinitarian formula the Nicene Creed had left unfinished regarding the Holy Spirit. His position was challenged on a technicality, that canon law forbade transferring a bishop between sees and he had never properly resigned Sasima, and rather than fight it Gregory resigned the archbishopric in the middle of the council itself and returned to Cappadocia to live out his remaining years in retirement, writing poetry, including an unusual autobiographical poem in verse, and letters, until his death around 390.