Gregory of Nazianzus
"The Theologian" — who completed what Nicaea began, settled the Spirit's divinity, and gave the church its most important argument about the incarnation
Gregory of Nazianzus (c. AD 329–390) is one of only three people in the history of the Eastern church to whom the unqualified title "The Theologian" has been given — the other two being John the Apostle and Symeon the New Theologian. The title was given to Gregory because he finished the work that the Council of Nicaea (325) began but left incomplete. Nicaea settled the question of the Son: homoousios, of the same substance as the Father, not a creature. It said almost nothing about the Holy Spirit. By the 360s the Arian controversy had mutated: some accepted the Son's full divinity while denying the Spirit's, arguing that the Spirit was a creature subordinate to both. These were called Macedonians or Pneumatomachians — "fighters against the Spirit." Gregory answered them with five orations delivered in Constantinople in 380–381, the most concentrated piece of Trinitarian theology the ancient church produced, and the Council of Constantinople (381) used his work to produce what we now call the Nicene Creed in its final form, with the article on the Holy Spirit fully expanded.
The Theological Orations
Gregory arrived in Constantinople in 379 to lead the small Nicene congregation there, at a time when the city was dominated by Arian and Macedonian bishops. He began preaching the five orations in a chapel in his cousin's house, which the orthodox nicknamed Anastasia — "Resurrection" — because through it the orthodox faith in Constantinople was being raised back from near-death. The orations covered, in order: the proper manner of theological discourse (Oration 27), the nature of God (Oration 28), the Son (Oration 29), more on the Son (Oration 30), and the Holy Spirit (Oration 31). This last oration makes the case for the Spirit's full divinity with an argument from the internal logic of the Godhead:
"If ever there was a time when the Father was not, then there was a time when the Son was not. If ever there was a time when the Son was not, then there was a time when the Spirit was not. If the One was from the beginning, then the Three were so too. If you throw down the One, I am bold to assert that you do not set up the other Two. For what profit is there in an imperfect Godhead? Or rather, what Godhead can there be if It is not perfect? And how can that be perfect which lacks something of perfection?"
Gregory of Nazianzus, Fifth Theological Oration (Oration 31), §4 · c. AD 380The argument is elegant and hard to escape. If the Spirit is not from the beginning — if there was a time when the Spirit was not — then the Godhead was, at that time, imperfect: lacking holiness, lacking the fullness of divine life. But an imperfect God is a contradiction in terms. The Spirit must therefore be co-eternal with the Father and the Son, of the same substance, not a creature. The Council of Constantinople (381) agreed, and the Niceno-Constantinopolitan Creed — the creed still confessed by Catholic, Orthodox, and most Protestant churches — expanded its article on the Spirit to say that he "proceeds from the Father, who with the Father and the Son together is worshipped and glorified."
What is not assumed is not healed
Gregory's second most important contribution came in his letters against Apollinaris of Laodicea. Apollinaris, trying to preserve the unity of Christ's person, had argued that the divine Logos replaced the human rational soul in Jesus — that Christ had a human body but not a human mind. Gregory's response, written in his letter to the priest Cledonius around AD 382, became the most quoted sentence in all of Christology:
"If anyone has put his trust in Him as a Man without a human mind, he is really bereft of mind, and quite unworthy of salvation. For that which He has not assumed He has not healed; but that which is united to His Godhead is also saved. If only half Adam fell, then that which Christ assumes and saves may be half also; but if the whole of his nature fell, it must be united to the whole nature of Him that was begotten, and so escape as a whole from sin."
Gregory of Nazianzus, Letter 101 (To Cledonius) · c. AD 382The logic is as compact as a mathematical proof. The fall corrupted the whole human person — body and mind. Salvation requires the whole human person to be united with God. If Christ did not take on a human mind, then the human mind was not united to God in him, and the human mind is not saved. The completeness of the incarnation — fully divine, fully human, lacking nothing of either — is not a theological luxury but a soteriological necessity. What is not assumed is not healed. The Council of Constantinople ratified this too; the church rejected Apollinarianism and affirmed that Christ was fully human in soul and mind as well as body.
The retreat and the poetry
Gregory was briefly president of the Council of Constantinople in 381, then resigned almost immediately under pressure from factions who disputed his canonical standing. He had never wanted to be in Constantinople; he had been sent there by Basil of Caesarea, who was his closest friend, and he found the city's theological politics exhausting and distasteful. He retired to his hometown of Nazianzus in Cappadocia and spent his last years writing poetry — a large body of verse, much of it theological, some of it personal and self-reflective, that is among the most humanly direct writing to survive from the ancient church. He was not only the rigorous theologian of the orations; he was also the man who could write about his own loneliness and the grief of watching friends die, in verse that still reads as recognizably human.
Where this lands
Gregory is in this record for two contributions that matter beyond academic church history. First: the Trinity is not an optional refinement that can be set aside for the simpler notion of "the Father and Jesus." If you take away the co-eternal Spirit, you take away the eternal fullness of God — you have a God who was once less than he is now, which is not the God of the Bible. The Spirit was not an addition; the Three were from the beginning. Second: the incarnation must be total. A Christ who became human in appearance only, or in body but not in mind, cannot save the whole human person. What he did not take, he did not heal. The full humanity of Jesus is not a limitation on his divinity; it is the mechanism of our salvation.
Related: Athanasius (the Nicene settlement, the Son), The Trinity, What the Early Church Confessed, The Word Made Flesh, The Witnesses, The Church, One Story. Sources: Gregory of Nazianzus, The Five Theological Orations and Letter 101 (public-domain translations, Nicene and Post-Nicene Fathers series, Schaff, Philip Schaff, ed.). Scripture from the King James Version, linked to BibleHub.