Gregory was the third of the great trio of Cappadocian Fathers — younger brother of Basil the Great, friend of Gregory of Nazianzus, brother and pupil of Macrina. Married briefly in his youth (his wife Theosebeia died), he became bishop of Nyssa at his brother Basil's appointment in 372.
He was the first Christian theologian to develop systematically the doctrine of infinite human growth into God — the 'epektasis' (perpetual reaching forward into infinity) of the soul's progress in the Christian life. His mystical works — the Life of Moses, On the Soul and the Resurrection (in which his sister Macrina speaks), the Great Catechism — established the foundations of Greek and Latin mystical theology, anticipating Pseudo-Dionysius, Maximus the Confessor, and ultimately John of the Cross.
At the First Council of Constantinople (381) he was a principal architect of the trinitarian theology of the Nicene-Constantinopolitan Creed. The bishops named him a 'pillar of Orthodoxy.' Pope Francis in 2024 began the formal process of declaring him a Doctor of the Church.