Ecumenical discussion today comes back again and again to the anxieties of the divided churches about their survival as ecclesial entities in a future united Church. These anxieties have a long history, but they have become newly acute in the ecumenical climate of the second half of the twentieth century. The history of these concerns is not always familiar to those engaged in today's debates, and that seems a good reason for writing a book about it.

On the history rides a number of ecclesiological concepts and assumptions; it also has a good deal to teach us about the problems we face in seeking to recognise a common faith or order in a future united Church, and in learning to make decisions together as Christians. Above all, the past, which has on the whole not always thought in terms of ‘ecclesial communion’, now has to be understood in a present where there is comprehensive exploration of the nature of communion.

It is important to try to understand the historical background because ideas are altered by being taken out of their context and they can persist unhelpfully after the times which partly justified them and created division. That obvious usefulness of history does not in itself resolve a profound methodological difficulty. Although theological principles can be seen at work in events, history is not theology.