Gregory was born around the year 335 into the great Christian family of Cappadocia, in the Roman East, today central Turkey, the younger brother of Basil the Great and of their remarkable elder sister Macrina the Younger, who shaped them both. Unlike Basil, who studied at Athens, Gregory was schooled at home and set out first on a worldly path, as a teacher of rhetoric. The church drew him in slowly, and around the year 372 his brother Basil, locked in a struggle to defend the faith of Nicaea, made him bishop of Nyssa, an obscure little town, partly to gain an ally in the fight. Gregory was no politician, and under the Arian emperor Valens he was accused, deposed by a hostile synod, and driven into exile; only when Valens died was he restored to his see.

What he lacked in worldly skill he more than made for in depth of mind. After Basil's death in 379 Gregory stepped into the front rank of the church's teachers, and at the Council of Constantinople in 381, the council that completed the Nicene Creed, he was among those who secured the confession of the full divinity of the Holy Spirit. The quarrel of that age can look like hairsplitting from a distance, but everything hung on it: if the Son and the Spirit were less than God, then it was not God who had come to save us, and the whole gospel failed. Gregory gave the church some of its surest language for holding that God is truly three and truly one.

But his genius ran deeper than controversy. He saw, more clearly than almost anyone before him, that the infinite God can never be possessed or mastered by the mind, that every concept we form of God, if we mistake it for God himself, becomes an idol; the modern theologian Jürgen Moltmann famously compressed Gregory's teaching into the saying that concepts create idols and only wonder comprehends anything. And because God is infinite, Gregory taught, the soul's journey into him never ends: there is no point at which we arrive and stop, only an endless growing from glory to glory, a doctrine he called perpetual progress and unfolded in his mystical reading of the Life of Moses. The soul that has come furthest is the one still most hungry for God. Even the resurrection he understood as a homecoming and a healing, "the re-constitution of our nature in its original form," in the words of his treatise On the Making of Man, translated by Henry Austin Wilson.

He died around the year 395, at the height of his powers, having outlived the brother and sister who taught him and carried their faith further than either. Where Basil built and governed and Macrina prayed, Gregory thought, and gave the church a vision of the Christian life as an unending ascent into a love that is always larger than we have yet found, a God we shall be discovering, joyfully, forever.