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. In this regard, Gregory is the first to use the idea of "procession" 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." Although Gregory does not fully develop the concept, the idea of procession would shape most later thought about the Holy Spirit.