@ebrown p,
Well, it certainly is true that not only the Roman Catholic Church but also the Eastern Orthodox Churches, the Anglican Church, and a variety of national and other churches claim to be members of the holy catholic church.
Most - if not all- of those I mentioned above are actually in full communion with the Bishop of Rome, the Pope.
Your claim about protestant of Luther's day is a bit surprising for me - do you have any source for that? (The monastery in Erfurt, where Luther hold his first mass as a Catholic, Roman Catholic, priest still is a monastery, a protestant monastery. And the nuns there ... have the very same rituals as catholic nuns. [Nowadays the "St Augustine's Evangelical Monastery", in Luther's days without the 'Evangelical'.)
[The Augustine, btw, first spoke about the church as "Catholic". But it was St. Vincent of LĂ©rins in his Commonitoria (434) who coined the term first.]
The
confusion is in my opinion, because various groups that have been condemned by the Roman Catholic Church as heretical or schismatic never retreated from their own claim to catholicity.