Here’s the abstract for my paper on Gregory of Nazianzen’s understanding of Trinitarian Relationship
Gregory Nazianzen used very specific terms to demonstrate the deity of the Son and Spirit by showing their relationship to the Father. He based his proof in the unquestioned deity of the Father, and then used relational terminology to explain how the Son and Spirit both shared in the deity of the Father and were distinct persons in and of themselves. The two terms which he primarily used to do this were begotten and procession. These terms were biblical and expressed the essence of their relationship, yet Gregory drew a sharp distinction between the two. Begetting is not procession and vice versa. To interchange the two would invalidate only-begotten and create either 2 Sons of a Son/Grandson relationship. But in his use of Begotten and procession Gregory described the Father as the Source of their divine nature and simultaneously refuted Sabellian Monarchism by establishing the presence of separate persons within the Godhead.