Of all the saints of the Church, Saint Gregory is one of the very few who was glorified with the title "theologian" on a par with the Holy Evangelist Apostle John. In the hymns of the Orthodox Church, he is praised for having defeated the sophistry of heretic wisdom and doing so with great eloquence, mastery and dedication. He left a rich heritage of philosophical works and religious poetry, prized not just among Christians, but also among secular philosophers.
He contributed to the development of the Church doctrine in multiple ways. First, he advanced the teaching of the Holy Trinity, by asserting that the three persons of the Triune God had a distinct essence not reduced to the essence of any other person of the Trinity. He also asserted the possibility of knowing the essence of God not by worldly reasoning, but by achieving oneness with Him through deification. He argued that the purity of the soul and body were more essential to knowing God and contemplating Him properly than an able mind.
His teachings and works supported the unity of the Church at a time when it was ravaged by influential and powerful heresies. When he returned from Athens, he found that even his father, a bishop, had fallen under the influence of Arianism. He spent much time and effort to convince him of the detrimental nature of Arianism and strengthen him in the faith. Heretics were also predominant in the capital of Byzantine, Constantinople. He preached incessantly, and more people were returning to the true faith. On the night of Pascha, a mob of armed heretics burst into the church and cast stones at the Orthodox, killing one bishop and wounding Saint Gregory. Yet the saint continued to preach, humbly and mildly, and his words softened hearts and returned many heretics into the fold of the Orthodox Church.
His legacy includes five theological discourses and some 45 sermons and was highly influential on Latin and Orthodox theological thought.