Abstract
This book, titled Gregory of Nazianzus and the Matter of the Deity of the Holy Spirit, which sheds light on the formation process of Christian theology, was written by Lecturer Dr. Elif Tokay. In this research, the author focuses on the role of Gregory of Nazianzus, one of the Cappadocian Fathers, in the systematization of the Trinity doctrine as "one ousia-three hypostasis". In this context, it is emphasized in the research that Gregory has been an effective theologian in the confirmation of the divinity of the Holy Spirit. From this point of view, it is underlined that the doctrine of the Trinity is determined through a gradual process. In this regard, besides the basic theological debates in the early periods of Christianity, various religious groups that were active at that time and the theological interpretations of these groups are also touched upon. Another theme emphasized here is the concept of "theosis" that Gregory of Nazianzus put forward in a mystical and soteriological context and the theological views he systematized within this framework.