The epistle To Peter on the Divine Ousia and Hypostasis has long been attributed to St Basil of Caesarea; but during the past century patristic scholars have come to believe that it probably was composed by St Gregory of Nyssa, Basil’s brother. If true, then the letter is a good place to begin our reflections on the trinitarian theology of Gregory. In it he elucidates the distinction for which the Cappadocians are famous—the distinction between hypostasis and ousia (see “The Search for Hypostasis“). But as we reflect on this letter and some of Gregory’s other writings, it’s important to keep in mind that while their formulation of the hypostasis/ousia distinction was helpful to the Church in achieving terminological consensus in trinitarian discourse, the Cappadocians were not consumed with the philosophical question of how the One and Three might be reconciled. Khaled Anatolios provides a much-needed caveat:
It has been my premise throughout this study that the language of hypostasis and ousia has been saddled with a disproportionate share in the burden of carrying the intelligibility of trinitarian doctrine. This language has been traditionally vaunted as the “Cappadocian resolution,” which ushered in the close of the “Arian” controversies by positing distinct terminology for what is one and what is three in God. … Such an approach makes it seem almost as if this terminological organization itself enabled trinitarian belief, as if everyone agreed that God was both three and one but just needed to find the appropriate terminology in order to subscribe to this belief. But, of course, hypostasis-ousia language and other terms denoting unity and distinction did not make it possible to believe that God is triune nor even to concretely conceptualize that belief. They were simply a posteriori logical-linguistic maneuvers that followed upon the belief concerning Father, Son, and Spirit that each is fully God and together they are one God. Linguistic frameworks demarcating unity and distinction are not the inner shrine of the meaning of trinitarian doctrine but a set of logical regulators that safeguard the contents of that meaning. The proper signification of hypostasis-ousia and kindred language is not to be found in its references to abstract logical categories of unity and difference but in its connections with the scriptural, liturgical, and soteriological conceptions and performances of how Father, Son, and Spirit are each fully God and together one God. (Retrieving Nicaea, p. 212)
The doctrine of the Holy Trinity is grounded upon the liturgical, catechetical, and ascetical life of the Church in which God is experienced as Father, Son, and Holy Spirit and cannot be abstracted from that life. When Basil and Gregory settled on the hypostasis/ousia terminology, they did not think they were explaining the metaphysics of divine triunity, and they certainly did not think they were inventing a new and different Deity. They understood, rather, that they were merely making explicit the grammatical rules that have always governed, however implicitly, churchly speech about the God of the Bible.