^^ You're right. "Christianity" can be anything that any christian wishes to say that it is (which can include arbitrarily saying that it is fundamentally only THREE things, taken from the bible, and not the OTHER things in the bible).

But with that level of vagueness or mutability, why say it's anything at all?

...thus, in any practical or real sense, it's "whatever anybody decided today to call it."

EDIT: But, in trying to have any sort of serious conversation (a conversation where something MEANS something), it's difficult to include elements that can mean almost anything--but which nevertheless are expected to be taken SERIOUSLY.