In the first section of this fourth chapter of Being as Communion Zizioulas traces the relationship between the “one” and the “many” in the eucharistic consciousness of the early Church. This idea of the incorporation of the “many” into the “one” is developed by the Apostle Paul (especially in 1 Corinthians 10:16-17) but can be traced back to earlier ideas of the figure of the “Servant of God” and the “Son of Man”.

But what is significant for us here is that this idea was from the beginning connected with the eucharistic consciousness of the Church. Paul in writing these words to the Corinthians, was simply echoing a conviction apparently widely spread in the primitive Church. (146)

The connection between the Servant of God imagery and the Eucharist continues in I Clement and in the Didache, while the imagery of the Son of Man is further developed in the eucharistic consciousness of John’s gospel.

It is significant that Christ appears here as the Son of Man, and not in another capacity, as he identifies himself with “the true bread.” Hence the eating of this bread is called specifically the eating of “the flesh of the Son of Man” who takes into himself every one who eats this bread, thus fulfilling his role as the corporate Son of Man. (146-147)

The ecclesiological consequences of this can be seen in the sources of the first three centuries. The fact that the Church comes to be called an ecclesia indicates a gathering together in a dynamic sense. Moreover, this gathering constitutes it as the whole Church.

… in the literature of the first three centuries at least, the local Church, starting again with Paul, was called the e0kklhsi/a tou= qeou or the “whole Church” or even the kaqolikh\ e0kklhsi/a and this not unrelated to the concrete eucharistic community. As the ecclesiology of Ignatius of Antioch makes clear, even the context in which the term kaqolikh\ e0kklhsi/a appears is a eucharistic one, in which Ignatius’ main concern was the unity of the eucharistic community. Instead of trying, therefore, to find the meaning of the “catholic Church” in this Ignatian text in a contrast between the “local” and the “universal,” we would be more faithful to the sources if we saw it in the light of the entire Ignatian ecclesiology, according to which the eucharistic community is “exactly the same as” (this is the meaning I would give to w3sper  which connects the two in the Ignatian text) the whole Church united in Christ. (148-149)

In this context, then, catholicity simply means the whole, fullness and totality of the Body of Christ as portrayed in the eucharistic community.