Continuing the discussion from yesterday about endonyms and exonyms, there's a religious take on that.
Lutheran theologians, for example, will sometimes refer to non-Lutheran Protestants as "the Reformed": clearly an exonym.
Many folks in the non-Lutheran group disagree with that assessment. Lots of Baptists, Methodists, some Anglicans, and others reject being called Reformed: they think of that term as meaning someone who is a Calvinist, whose theology -- and perhaps practice -- is Reformed, usually in accord with the Genevan Reformation in which John Calvin was active.
There's nothing wrong with Lutherans using this term among themselves, but it's not helpful in discussions with non-Lutherans. Some non-Lutherans find it insulting, and at the very least, it's confusing. Using a term isn't bad unless it's deliberately used to confuse an issue. Lutherans aren't doing that, but even among Lutherans, there should be a clear teaching as to what "Reformed" means -- in-group and out of the group.