John 17 has been preached again and again, stressing on the necessity of the unity of the church. But then perhaps it really depends on whether you are a Protestant or a Roman Catholic when you read such texts. A Protestant tends to read it as a kind of spiritual unity, while a Roman Catholic would read it as an organizational unity. That’s what makes them Protestants or Roman Catholics in the first place, anyway.
A (perhaps not so) recent dialogue about unity and diversity in New Testament ecclesiology was conducted in the Fourth World Conference on Faith and Order in 1963. It was between Ernst Käsemann, a Protestant scholar, and Fr. Raymond Brown, a Roman Catholic scholar. And, guess what. Käsemann stressed the diversity of the early church, while Fr. Brown stressed the unity.
To put it in other words, a Protestant tends to find a diversity in the New Testament, while a Roman Catholic would find a united voice. And both do exist side by side in our Scriptures. It is easy to see how the early church is a diverse organism and since from its very beginning you could find multiple voices within it: Judaizer controversy (Acts 15, Gal 1), schisms in Corinth, heretics in Colossae, etc.
But it is quite obvious as well how the early church came to be organized into a similar pattern with a few offices like deacons and elders (the Pastoral Letters). Indeed, as early as the beginning of the second century, Ignatius of Antioch wrote that “wherever the bishop appears, let the people be there; just as wherever Jesus Christ is, there is the Catholic Church.” (Letter to the Symrneans 8.2, ca. 110 AD) Two things to be highlighted here. First, the office of a bishop was assumed in the days of Ignatius. It was assumed that the church would be centred around a bishop. A bishop represented the local church. Second, Ignatius coined the term ‘catholic’ to denote the whole church. From then on, the church is the Catholic church.
But, even then, catholicity or universality of the church was actually defined against the so-called heretics, since Ignatius was writing against them. He contended that the heretics taught different teachings according to their own, but the Catholic church, wherever you found it, taught the same (if you think about it, Ignatius could easily apply the same reasonings to the Protestant churches nowadays!). So even catholicity or universality needs heretics, the other side, in order to define itself.
So perhaps it isn’t really straightforward to define what does it mean by the unity of the church. A good litmus test might be whether we are willing to worship and break bread together with or in the other churches. My father always said that wherever you go, if you go to a Catholic church, you will find an identical liturgy and hence basically you can worship anywhere. And I guess it contains some grains of truth. We, the Protestants, tend to back off easily if we did not feel comfortable in a church or a fellowship which we attended. We are way too easy to break fellowship with one another and build our own brand of church or fellowship.
Although, on the other hand, one could immediately field an objection: Isn’t local expression of faith to be encouraged and fostered? Why uniformity when we could celebrate diversity? The problem is, diversity is often only a mask of local uniformities.