The position I have heard most often is just the opposite, that Christians are Christians. My Sunday School class is a mix of former Catholics, former Baptists, former Presbyterians, and other crossovers, and no one is particularly concerned about those details as meaning they were not real Christians. In public, my experience has been the same. The bottom line: I do not see the problem you think you see.
I think that his view of the problem is overly influenced by reading FR religion threads. It's why I seldom look at them anymore.
But Talisker, the rest of the world is not like that, at least not the rest of the world I know.