To Be

Up until the period when the Cappadocians undertook to develop a solution to the trinitarian problems, an identifying of ousia with hypostasis implied that a thing’s concrete individuality (hypostasis) means simply that it is (i.e. its ousia). Now, however, changes occurred. The term hypostasis was dissociated from that of ousia and became identified with that of prosopon. But this latter term is relational, and was so when adopted in trinitarian theology. This meant that from now on a relational term entered into ontology and, conversely, that an ontological category such as hypostasis entered the relational categories of existence. To be and to be in relation becomes identical. For someone or something to be, two things are simultaneously needed: being itself (hypostasis) and being in relation (i.e. being a person). It is only in relationship that identity appears as having an ontological significance, and if any relationship did not imply such an ontologically meaningful identity, then it would be no relationship. Here is certainly an ontology derived from the being of God.
Being as Communion, John Zizioulas