Saint Gregory's ascetical life, his continence, purity, and lack of covetousness aroused the envy of his conceited and sin-loving peers, pagans that they were, and they decided to slander Saint Gregory. Once, when he was conversing with philosophers and teachers in the city square, a notorious harlot came up to him and demanded payment for a sin he had supposedly committed with her. At first Saint Gregory gently remonstrated with her, saying that perhaps she had mistaken him for someone else. But the profligate woman would not be silenced. Then he asked a friend to give her the money. Just as the woman took the unjustified payment, she fell to the ground in a demonic fit, and the fraud was revealed. Saint Gregory prayed over her, and the demon was expelled. This was the first of his miracles.
 

After returning to Neocaesarea, the Saint fled from worldly affairs, into which influential townsmen persistently sought to push him. He went into the desert, where by fasting and prayer he attained great spiritual heights, as well as the gifts of clairvoyance and prophecy. Saint Gregory loved his life in the wilderness and wanted to remain in solitude until the end of his days, but the Lord willed otherwise.

Learning of Saint Gregory’s ascetical life, Bishop Phaidemos (Phaίdēmos) of the Cappadocian city of Amaseia, decided to make him Bishop of Neocaesarea. But foreseeing in spirit the intention of Bishop Phaίdēmos, the Saint hid himself from the hierarch's messengers who were sent to find him. Then Bishop Phaidemos consecrated Saint in absentia as Bishop of Neocaesarea, entreating the Lord to bless the unusual ordination. Saint Gregory regarded the extraordinary event as a manifestation of God's will, and he did not dare to protest. This episode in the life of Saint Gregory was recorded by Saint Gregory of Nyssa (January 10). He relates that Saint Gregory of Neocaesarea received the episcopal rank only after Bishop Phaidemos had ordained him to all the canonical ranks.

During this time, the heresy of Sabellius and Paul of Samosata began to spread. They taught falsely concerning the Holy Trinity. Saint Gregory prayed fervently and diligently imploring God and His most pure Mother to reveal the truth to him. The Most Holy Theotokos appeared to him, as radiant as the sun, and with her was the Apostle John the Theologian dressed in hierarchal vestments.

By the command of the Mother of God, the Apostle John taught the Saint the correct way to speak of the Mystery of the Most Holy Trinity. Saint Gregory wrote down all that Saint John revealed to him. The Symbol of the Faith, as written down by Saint Gregory, is a great divine revelation in the history of the Church. The teaching concerning the Holy Trinity in Orthodox Theology is based on it. Subsequently, it was accepted by the Holy Fathers of the Church: Basil the Great, Gregory the Theologian, and Gregory of Nyssa. Saint Gregory's Symbol (Creed) was later examined and affirmed in the year 325 by the First Ecumenical Council, showing its enduring significance for Orthodoxy. Even those who disagreed with Saint Gregory regarded him as a second Moses.2

After becoming a hierarch, Saint Gregory journeyed to Neocaesarea. Along his way from Amaseia, he cast out the demons from a pagan temple, the priest of which he converted to Christ. That convert was a witness to yet another of the Saint's miracles: at his word a large stone moved from its place.

The Saint's sermons were direct, lively and fruitful. He taught and worked miracles in the name of Christ: he healed the sick, helped the needy, and settled disputes and complaints. Two brothers who shared an inheritance were unable to agree about their dead father's property. A large lake was the cause of their dispute, for each brother wanted the lake for himself. Both of them gathered their friends together, and were ready to come to blows. Saint Gregory persuaded them to delay their fight until the following day, while he prayed all night long on the shore of the lake which had sparked the quarrel. When dawn came, everyone saw that the lake had dried up or gone underground. Now, by the Saint's intense prayer, there was only a stream, and its course defined the boundary line. Another time, during the construction of a church, he commanded a hill to move and make room for the foundation to be dug.
 

When the persecution of Christians began under Emperor Decius (249-251), Saint Gregory led his flock to a faraway mountain. A certain pagan, who knew where the Christians were hiding, informed the persecutors, and soldiers surrounded the mountain. The Saint went out into an open place, raised his hands to heaven, and ordered his deacon to do the same. The soldiers searched the entire mountain, and several times they went right past those who were praying. Unable to see them, they gave up and went away. In the city they reported that there was nowhere to hide on the mountain. There were no people, just two trees standing next to each other. The informer was struck with amazement, he repented of his ways and became a devout Christian.

Saint Gregory returned to Neocaesarea after the end of the persecution. With his blessing, Church Feasts were established in honor of the martyrs who had suffered for Christ.

By the holiness of his life, his effective preaching, his miracles, and inspired guidance of his flock, the Saint increased the number of converts to Christ. When Saint Gregory first came to his See, there were only seventeen Christians in Neocaesarea. At the time of his repose, only seventeen pagans were left in the city.