Before we examine the question of how to formulate a theology of Christian unity, it would be helpful to know what exactly is at stake. What are the benefits of ecumenism that its supporters are trying to embrace? What are the dangers that its critics are trying to avoid?
The most helpful way I know of to think of these issues is to set them in a continuum with truth on one end and love on the other. Those at the “love” extreme view doctrinal correctness as nearly irrelevant. This is the “I'm ok, you're ok, God is love so let's just hold hands” attitude of liberal theology. On the other end are those who tolerate no deviation whatsoever from their understanding of Christian theology. Those who err on the side of love anathematize no one, those who err on the side of truth anathematize everyone.
Both extremes are clearly in error, but there is a lot of gray area between them. How wrong is too wrong? At what point does an error become heresy? An offense against God's truth? Worth breaking fellowship over? These are difficult questions, and there are a lot of answers to them. Any stance is liable to be criticized for being either too strict or too lax. Nevertheless, a line must be drawn somewhere. Which element, zealous defense of God's truth or loving inclusion of differing interpretations, deserves more consideration?
The dangers of erring too far on the side of love and inclusion are, I think, sufficiently obvious. Christianity is and always has been fundamentally creedal. It is grounded on certain truths, expressed in the early creeds and elsewhere, without which it cannot stand.
The dangers of falling too far on the side of truth may not be as obvious, particularly to most Protestants. What is so important about Christian unity that it would justify any compromise on doctrine at all? The importance of unity lies in the fact that schism is not an act without negative consequences. It is divorce. It is always a tragedy, and the benefits of doctrinal purity must always be weighed against the damage of schism.
The importance of unity among believers is given a lot of emphasis in the Bible. In John 17, the only place in the gospels where Jesus prays specifically for the Church (those who will believe through the apostle's testimony), His one request is this: “that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me.” The call to unity is high: the Church's oneness is to mirror the eternal oneness of the trinity. The benefits of unity are also high: oneness among believers gives credibility in the eyes of the world to Jesus' identity.
We are no closer to a solution, however. It would be ideal to never have to decide which evil, schism or heresy, was the lesser. But that is not the position we find ourselves in. We find ourselves part of a Church assailed by both. How do we live in such a Church? How do we value both doctrine and unity, as best we can, in an world constantly demanding us to choose between the two?