This question has troubled Christians, East and West, for a thousand years or more. In fact, if history is read without undue bias it has quite frankly troubled the church since the first century and beyond. There are several ways to come at it.
First, you can argue that there can only be one “visible” communion (with its beliefs, practices, leadership, sacraments, etc.) that is the true church. Both the Roman Catholic and Orthodox Church arrive at something like this position through a series of well-argued points and conclude that they are in the one “true” church. The Orthodox actually do not speak about “true” as opposed to false so much as they say they know where the “church is but not necessarily where it is not.” Either way, both these great church bodies believe they are the continuous, one, catholic and true Christian church. If I personally believed this then I would begin the conversion process today. Obviously, to the disappointment of some good friends on both sides, I am not in the process of converting. I have been asked, “Have you ever considered converting?” The answer is “Yes.” In fact, by considering this question on a deeply personal basis I have seriously been forced to understand my own position, and why I still hold it, and that of the Catholic and Orthodox Church as well. I do not pretend to understand this question completely but I have sought to act in good faith based upon my current understanding and conscience. I hope I will continue to do this throughout my life. This is why I can never say what I would do tomorrow or the next day. I do hope I will keep listening to Christ’s Word and the Holy Spirit. If this makes someone uncomfortable I simply cannot worry about it. I love Christ and I love the church so I want to be as faithful as I can be in my journey home.
Second, you can argue as I do that the “visible” church consists of a number of ecclesial communities and most of them are valid churches. I do not believe, as some very conservative Protestants, that the Roman Catholic Church is not a church. I find this argument rather absurd the more I consider it. (I grew up hearing it so I have heard and read a great deal of it over the course of a long life.) I also believe churches may teach or believe a number of things badly and that their practice might be lacking in solid biblical support but they are still churches nonetheless. When I consider the church in Corinth I think I can see why this is true.
So if many visible congregations are real churches, at least so long as they confess Christ and the core of orthodoxy, then is there really one church as The Apostles’ Creed puts it? I believe the answer is an unqualified yes. In this week’s You Tube clip on my book I discuss this briefly.