Theological and other works[edit]
Gregory's most significant theological contributions arose from his defense of the doctrine of the Trinity. He is especially noted for his contributions to the field of pneumatology—that is, theology concerning the nature of the Holy Spirit.[15] In this regard, Gregory is the first to use the idea ofprocession to describe the relationship between the Spirit and the Godhead: "The Holy Spirit is truly Spirit, coming forth from the Father indeed but not after the manner of the Son, for it is not by generation but by procession, since I must coin a word for the sake of clearness."[16] Although Gregory does not fully develop the concept, the idea of procession would shape most later thought about the Holy Spirit.[17]
He emphasized that Jesus did not cease to be God when he became a man, nor did he lose any of his divine attributes when he took on human nature. Furthermore, Gregory asserted that Christ was fully human, including a full human soul. He also proclaimed the eternality of the Holy Spirit, saying that the Holy Spirit's actions were somewhat hidden in the Old Testament but much clearer since the ascension of Jesus into Heaven and the descent of the Holy Spirit at the feast of Pentecost.
In contrast to the Neo-Arian belief that the Son is anomoios, or "unlike" the Father, and with the Semi-Arian assertion that the Son is homoiousios, or "like" the Father, Gregory and his fellow Cappadocians maintained the Nicaean doctrine of homoousia, or consubstantiality of the Son with the Father.[18]:9,10 The Cappadocian Fathers asserted that God's nature is unknowable to man; helped to develop the framework of hypostases, or three persons united in a single Godhead; illustrated how Jesus is the eikon of the Father; and explained the concept of theosis, the belief that all Christians can be assimilated with God in "imitation of the incarnate Son as the divine model."[18]:10
Some of Gregory's theological writings suggest that, like his friend Gregory of Nyssa, he may have supported some form of the doctrine ofapocatastasis, the belief that God will bring all of creation into harmony with the Kingdom of Heaven.[19] This led some late-nineteenth centuryChristian universalists, notably J. W. Hanson and Philip Schaff, to describe Gregory's theology as universalist.[20] This view of Gregory is also held by some modern theologians, such as John Sachs who said that Gregory had "leanings" toward apocatastasis, but in a "cautious, undogmatic" way.[21] However, it is not clear or universally accepted that Gregory held to the doctrine of apocatastasis.[22]
Apart from the several theological discourses, Gregory was also one of the most important early Christian men of letters, a very accomplished orator, perhaps one of the greatest of his time,[18]:21 and also a very prolific poet, writing several poems with theological and moral matter and some with biographical content.