The Most Reverend John Zizioulas (1931-) attended the Universities of Thessaloniki and Athens and then Harvard Divinity School where he studied patristics under the famous Georges Florovsky. For 14 years he served as professor of theology at the Universities of Glasgow and Edinburgh, and has been a visiting professor at the University of Geneva, Gregorian University, and King's College, London. In 1986 he was consecrated as a bishop and named Metropolitan of Pergamon.
John Zizioulas' influence goes far beyond the Orthodox Church. He has impacted theological thinking in Roman Catholic, Anglican and Protestant circles as well. Many regard his 1985 book, Being As Communion: Studies in Personhood and the Church, to be the most significant theological work of the late 20th century.
The main themes of Zizioulas' work are freedom and otherness, both human and divine. It has been said that "grounding his work in the Cappadocian Fathers and St Maximus Confessor in particular, he articulates a relational ontology in which neither unity nor plurality have priority" (See OrthodoxWiki).
The following paragraphs are from Being As Communion, 1997:
“The Church is catholic, not because she is obedient to Christ, i.e. because she does certain things or behaves in a certain way. She is catholic first of all because she is the body of Christ. Her catholicity depends not on herself but on Him. She is catholic because she is where Christ is. We cannot understand catholicity as an ecclesiological notation unless we understand it as a Christological reality." (page 158)
"The Christological character of catholicity lies in the fact that the Church is catholic not as a community which aims at a certain ethical achievement (being open, serving the world, etc.) but as a community which experiences and reveals the unity of all creation insofar as this unity constitutes a reality in the person of Christ. To be sure, this experience and this revelation involve a certain catholic ethos. But there is no autonomous catholicity, no catholic ethos that can be understood in itself. It is Christ’s unity and it is His catholicity that the Church reveals in her being catholic. This means that her catholicity is neither an objective gift to be possessed nor an objective order to be fulfilled, but rather a presence, a presence which unites unto a single existential reality both what is given and what is demanded, the presence of Him who sums up in Himself the community and the entire creation by His being existentially involved in both of them. The Church is catholic only by virtue of her being where this presence is.” (page 159)
"In the celebration of the eucharist, the Church very early realized that in order for the eucharistic community to become or reveal in itself the wholeness of the Body of Christ (a wholeness that would include not only humanity but the entire creation), the descent of the Holy Spirit upon this creation would be necessary. The offering up of the gifts and the whole community to the throne of God, the realization of the unity of the Body of Christ, was therefore preceded by the invocation of the Holy Spirit." (page 160)
"This means not only that human attempts at “togetherness,” “openness,” etc., cannot constitute the catholicity of the Church, but that no plan for a progressive movement towards catholicity can be achieved on a purely historical and sociological level. The eucharistic community constitutes a sign of the fact that the eschaton can only break through history but never be identified with it. Its call to catholicity is a call not to a progressive conquest of the world but to a “kenotic” experience of the fight with the anticatholic demonic powers and a continuous dependence upon the Lord and His Spirit. A catholic Church in the world, cognizant as she may be of Christ’s victory over Satan, lives in humility and service and above all in constant prayer and worship." (pages 161-162)
"In such a catholic outlook the entire problem of the relationship of the Church to the world receives a different perspective. The separation and juxtaposition of the two can have no essential meaning because there is no point where the limits of the Church can be objectively and finally drawn. There is a constant interrelation between the Church and the world, the world being God’s creation and never ceasing to belong to Him and the Church being the community which through the descent of the Holy Spirit transcends in herself the world and offers it to God in the eucharist." (page 162)