Gregory and Basil both spent much effort defending the Faith against the attacks of the [[Arianism|Arians]]. He was twice deposed as leader of from his See see because of false accusations made by the heretics. His position as bishop He was finally restored in 378.

The next year, 379, his brother [[Basil the Great]] died. As the two were extremely close, Gregory was very grieved at his loss. To honor his brother, Gregory wrote his funeral oration and then completed Basil's ''Hexaemeron''("Six Days"), a series of nine sermons, delivered during [[Great Lent]], which described and elaborated upon the [[Genesis]] account of the world's creation in six days (Hexaemeron means "six days"). The following year, Gregory's sister [[Macrina the Younger|Macrina]] also died, and Gregory wrote a [[hagiography]] detailing her life.

About this time Gregory attended the Council of Antioch, a local [[synod]], in which he zealously defended Orthodoxy. The council was called to rebut refute a [[heresy]] which denied the perpetual virginity of the [[Theotokos]], on one hand, and other the other hand forbid . The council also forbade worship of her as God or part of the Godhead. Gregory was simultaneously continuing to fight Arianism. Next, he He also attended the [[Second Ecumenical Council]] in Constantinople, which added the final section concerning the [[Holy Spirit]] to the [[Nicene-Constantinopolitan Creed]].