St. Gregory of Nazianzus, known by the Orthodox Church as St. Gregory the Theologian (?329 - ?390 AD), was the son of the Bishop of Nazianzus (Cappadocia).

St. Gregory received the best education available, at the University of Athens, where St. Basil, his lifelong friend, and Julian, the future emperor, were fellow-students. In 359 AD he left Athens and became a monk, living a solitary life with St. Basil at Pontus. After two years, St. Gregory returned home to help his aging father manage his diocese. Against his wishes he was ordained a priest and then fled to St. Basil for 10 weeks. He returned to his new duties and wrote an apologia, titled "Defence of the Flight to Pontos", saying that no one can undertake to shepherd the spiritual flock without becoming a temple of the living God, "a habitation of Christ in the Spirit". He also said, "It is necessary first to be purified, then to purify; to be made wise, then to make wise; to become light, then to enlighten; to approach God, then to bring others to Him; to be sanctified, then to sanctify". This treatise became a classic on the nature and duties of the priesthood.

After St. Basil became Archbishop of Caesarea, he had St. Gregory consecrated Bishop of Sasima, but St. Gregory continued to help his father with his duties. Following the death of his father in 374, St. Gregory lived a solitary life in Seleucia until about 380 AD.

After the death of the persecuting emperor Valens, peace returned to the Church, but Constantinople was dominated by Arians. Neighbouring Bishops sent for St. Gregory to restore Constantinople's Christian community. Protesting, he moved to Constantinople, where he preached his famous sermons on the Trinity. His reputation spread and his audience increased, but the Arians attacked him by slander, insults, and violence. He persisted in preaching the faith and doctrine of Nicea (later known as the Nicene-Constantinopolitan Creed). In 381 AD, the Council of Constantinople proclaimed the conclusions of Nicea as authentic Christian doctrine. During the council, St. Gregory was appointed Bishop of Constantinople and installed in the basilica of St Sophia. Opposition to him, however, continued. He resigned for the sake of peace after restoring Orthodoxy in the capital.

He returned to Cappadocia, which was still without a Bishop, where he administered the See until a successor was appointed in c. 384 AD. He then retired to his estates and spent his time reading and writing. His writing included religious poetry (later to become Orthodox hymns), his autobiography, epistles, essays and sermons. He died at Cappadocia.