While it is generally recognized that differences in ecclesiology are the ones that constitute the most stubborn obstacles to Christian reunion, there are good grounds for hope in the fact that most Christians do agree in professing their faith in "one, holy, catholic and apostolic church." Indeed, honest facing up to the contradiction between our common faith in the "one church" and the divided state of Christianity is a prime motive for the ecumenical movement.

It is true, of course, that the mere fact that Christians use the same words in professing their faith about the church does not eliminate their deep differences in ecclesiology. Profoundly divergent answers will be given when one asks: What do you mean when you say "the church"? and How do you understand it to be one, holy, catholic and apostolic? Ecumenical progress, then, calls for the effort to reach a common under-
standing of the faith we profess. Theology has an important role to play here, because theology is defined as "faith seeking understanding." Ecumenical dialogue is most fruitful when it is a concerted effort, on the part of people coming from different theological traditions, to seek a deeper understanding of the creed in which they already profess a common faith, in the hopes that such a deeper understanding will get beneath their differences to the common ground where they are at one.

An essential step in this process is for Christians to seek to deepen and clarify their understanding of their faith to the light of their own respective traditions. Such is the purpose of this book. What we are seeking here is an understanding of our profession of faith concerning the church, in the light of the Roman Catholic tradition.