“Does God exist because He has to exist? Does God exist simply because He exists? Is there causation in God’s being, which is something like an event—of course outside time—that brings about His being, or is it rather the dead ousianic tautology of something existing because it exists, the logic of the 'self-existent'?” These are the questions posed by Zizioulas who believed that they affect our way of looking at existence in general.

Zizioulas states the basic truth central to Christian theology and is based on the teachings found in the Bible. Christianity is distinguished from other monotheistic religions by its belief in the Trinity, and “although it professes faith in one God, it understands this God as existing in a Trinitarian way, i.e., as Father, Son, and Holy Spirit.” He identifies this Trinitarian formula of the Christian faith in Christian Baptism, which has always indicated that “faith in the Holy Trinity is not simply a matter of accepting a theoretical proposition about God, but of relating one’s existence to this faith.” Consequently, for Zizioulas, “Baptism in the Trinity means entering into a certain way of being which is that of the Trinitarian God. Trinitarian theology has profound existential consequences.”

Zizioulas begins with the following proposition, derived from the Greek fathers:

Nothing exists in itself—not even God.
All things exist in and by virtue of communion-including God.
Communion cannot exist in itself; yet is the basis of ontology.

Thus, Metropolitan John’s starting point is that “even God exists thanks to an event of communion.” The Trinity is thus not only the origin of being but the very condition of being.

However, Metropolitan John reminds us that “a person is not only relational but at the same time other. The Father as a Person is not the Son or the Spirit, the Son is not the Father or the Spirit, and the Spirit is also other than the Father and the Son. As divine nature or substance the three are one God. But as divine persons they are other.”

Let’s see more closely Zizioulas' Trinitarian theology. “In the patristic understanding of the person, the three Persons are distinguished from each other because (and while) they coexist. This coexistence, being eternal (the Father has the Son eternally, etc.), makes the existence of one Person without the other Persons inconceivable. Thus, what is 'common,' the essence, is not in contrast with the personal otherness.” From this springs the truth that “person and essence are not in conflict, as in the personalism of existentialism, and for this reason freedom is a positive and affirmative stance; in other words, it is identified with love.”

Zizioulas has stressed repeatedly that “the person cannot be conceived of without the essence, and the essence of God cannot be conceived of 'in a naked state,' without the person.”

The foundation and guarantee of the unity of God’s being for Zizioulas is not simply the commonality of one essence (ἡ κοινοτης τῆς μιᾶς οὐσίας), but the fact that it is the essence of the One and God the Father, Who eternally (ἀϊδίως) gives birth to the Son and eternally (ἀϊδίως) proceeds the Holy Spirit. He says: “The co-emergence of divine nature with the Trinitarian existence initiated by the Father implies that the Father, too, ‘acquires,’ so to speak, deity only ‘as’ the Son and the Spirit are in existence (he is inconceivable as Father without them), that is, only ‘when’ divine nature is ‘possessed’ by all three.” For the Fathers of the Church, the name Father precedes as μεῖζον (“greater”), and it is by predominance the divine designation (θεωνυμία) of this causeless Person of the Holy Trinity. This is in accordance with St. Basil the Great: “Because the Father is the principle of the Son, the Father is greater as the cause and principle of the Son.”

But Zizioulas asks the following question: “Is the Father the only cause of divine being, of the Trinity? Often, these days, we confront such questions by invoking the concept of “consubstantiality,” since, according to Zizioulas “unity and diversity coincide in God’s very being.” He admits that his stress on the Cappadocian Fathers’ idea that the Father is the “cause” of the Trinitarian existence of God is “perhaps the most controversial point for many people”. According to Zizioulas, the fact that the causality of “how God is” clearly relates “only to the Person of the Father—and not to the shared essence” (he says that St. Gregory of Nyssa is clear on this point)—“does indeed make the Person a causal principle of the divine being.”

With this statement, Zizioulas displays the cornerstone of his theological perspective. The Cappadocian concept of the causality (“the cause of Trinitarian being refers to the person of the Father”) was introduced “in order to preclude any identification of the essence of God with the Father, that being the identification that the Eunomians tried to make so that they could then maintain that since the Son is not the Father, He lies outside the essence of the Father and so is not “consubstantial with the Father.”

However, Zizioulas holds that this truth is drawn not from the oikonomic acts, but from the liturgical experience. “It is characteristic that all the Anaphora prayers in the ancient Liturgies (St. Basil, St. John Chrysostom, etc.) are specifically addressed to the Father, something that unfortunately goes unnoticed by both our celebrants and our theologians.” In this liturgical approach to the Being of God Zizioulas finds the clearest and safest way of looking at God as an immanent (and not only economic) Trinity.

Regarding apophaticism and ontology, Zizioulas warns in a very insightful way that theology has nothing to say about the essence of God (“there we have total apophaticism”.) It does, however, have something to say about the personal existence of God, and that is that God comes near to us and is known and engaged in dialogue only as a Person. “We cannot pray either to the divine essence or to the divine energies.” In this sense, Dionysios Skliris rightly points out that the Metropolitan of Pergamon “insists on the use of the term ‘ontology,’ regarding God, criticizing apophatic theologies that disconnect ontology from God, considering Him as transcendent and beyond essence in a way that deprives the possibility of a theological ontology. And he even connects ontology not only with the divine essence, but also with the person, speaking of ‘ontology of the person,’ and denouncing those who identify ontology exclusively with the essence, and therefore consider the person as something non-ontological.” An apophatic theology enables us to go beyond the Economic Trinity and draw a sharp distinction between ontology and epistemology. Yet, Zizioulas stresses out that “the supraessential God of apophatic theology does not cease to be the ‘He who truly exists’ of the Divine Liturgy, and it is under precisely this identity that we address ourselves to Him.”

Particular characteristics of Zizioulas’ theological synthesis is the motif of “the one and the many” in the perfect communion where the one and the many are mutually constitutive: “there are no Son and Spirit without the Father, but equally there is no Father without the Son and the Spirit.” Equally important, according to McPartlan, this pattern of relations recurs throughout the Church, which participates in God’s life. Zizioulas is very affirmative about this: “There is no model for a proper relation between communion and otherness either for the Church or the human being other than the Trinitarian God. If the Church wants to be faithful to herself, she must try to mirror the communion and the otherness that exists in the Triune God. The same is true of the human being as the image of God.” The human person is called to fellowship and to reciprocal relationship with others. Just as God is a fellowship of Persons, so too the Church is a fellowship of persons. This paradox of unity and otherness found in the core of Trinitarian being also constitutes the essence of being a person.

To sum up, Zizioulas proceeds from the mystery of the Tri-hypostatic God that was revealed to the world in the divine revelation through Christ, that is, from the Holy Trinity of the divine Persons, who exist and live in the eternal communion of love, of being and life, of freedom and immortality. Man’s personal being reflects the Triune Communion of the Persons, and man can, therefore, realize his fullness only in Christ, in the Eucharistic union with his Body that, at the same time, is the communion, koinonia, with the Holy Spirit. Zizioulas counters this by emphasizing the patristic insight that otherness and communion are embedded in the Trinitarian framework, so the understanding of the tropos (mode) of Trinitarian life also directly affects the understanding of the tropos of human life. This communion of god-like, Christ-like persons is realized and fulfilled in the Eucharist, according to the Triune prototype, in the grace-filled and loving communion with Christ and with each other, because there is no communion without persons, nor can there be persons without communion. Several of his works are central to this subject. This perspective, inaugurated with “From the Mask to Person” (1977), reaffirmed in the Being and Communion, Communion and Otherness, The One and the Many, The Eucharistic Communion and the World, and blossoms in his final book Remembering the Future, which could serve as an all-encompassing model for the Christian theology.

His theological framework of Trinitarian theology of persons in communion has far-reaching implications and connections to various domains of theology. In a unique way he incorporates within it, as essential elements, all domains of theology: ecclesiology, Christology, Pneumatology, ethics, anthropology, cosmology, but also domains such as ecology.

Finally, let’s add here that by affirming that “nothing exists in itself—not even God,” and that “even God exists thanks to an event of communion,” Zizioulas mounts, according to Rowan Williams (Foreword to Communion and Otherness), “a formidable challenge to atheism by affirming very simply that it is meaningless to discuss ‘whether or not’ God exists in abstraction from the question of ‘how’ God exists. To ask whether God exists is really to ask about what the relations are that you can recognize yourself as involved in—because God is irreducibly a living complex of relation, Father, Son and Holy Spirit. But this ‘complex’ is not just a given plurality, it is the work of freedom—the Father's personal liberty and love generate the inseparable Other, the eternal Son, and 'breathe out' the eternal Spirit. The Father is never alone, nor is the Father simply one among three divine beings alongside each other; it is his absolute freedom to be completely for and in the Other that is the root and rationale of Trinitarian life. And this utter freedom for the Other becomes the insight that allows us to make sense of the freedom of creation, with all that this implies.”