Just like people who practice Catholicism aren't considered Christians.
That's exactly what they're considered. Catholicism is the most populous and self-proclaimed original "branch" of Christianity. The three primary and fundamental branches of Christianity are Catholicism, Protestantism, and Eastern Orthodoxy. Each of these has several, to hundreds+ of subcategories beneath them (some of the larger ones are mentioned by smeagol), and there are also other smaller branches that may or may not actually be reasonably classified as separate from the primary three branches aforementioned.
It's okay though, because you've definitely got the key takeaway from the whole religion (teachings of jesus, and old testiment - hebrew, jewish translations, etc)- it's all bullshit and silly.