God refers to the church as a mystery. What is this mystery? The mystery is that from the beginning God intended to gather His church from all nations and tribes. He wanted His church to be international (a universal church), and not limited to the tribe of Israel. This mystery was made clear in Christ and forms the heart of mission work.
When a church identifies herself as the only church which holds to the truth of scripture it is refered to as denominational isolationism. This article shows the dangers of denominational isolationism; namely, that it leads to separatism and doctrinal pride. Confessing one holy catholic church sets the church free from such isolationism.