Perhaps all Christians are members of the one true church. The Reformation certainly pointed out some terrible abuses and fallacies. Completely successful or not, The Counter-Reformation tried mightily to correct them. The Pope certainly would never write off Billy Graham, and vice-versa.
The Romans and the other Catholics say they base the Faith on the Bible AND the ever mysterious and often highly suspect Magisterium, which is briefly like the Supreme Court zealously honoring "precedent." Both lead to awkward moments. OTOH, what our Christian ancestors did and believed is vitally important to know.
The Nicene Creed contains the core of what all Christians believe. Of course, one may take issue with the formula.
Someday, all Catholics will feel the personal faith of the Protestants. And, the Protestants may come to evaluate more of the preserved tradition of the church left to us by the first Jewish converts and the various peoples of the Roman empire, whose official state religion Christianity in various forms came to be.