That's what we were thought in Baptist about the catholic.. though I never been a catholic before but I did herd about why there are canon and non-canonical and when I asked them these questions.. they read to me a passage that those books about new and old shouldn't increase more or be edited(don't really remember about the passage) so they told me that they just made those books up..
I didn't really care much.
In this country were much more of war in religion, and not just baptist and catholic.. some others as well like INC and Dating daan and muslims and probably some religions that you never herd of, some has reasonable reasons but some might be just want to get some money.
There is a KJV-only movement among some Baptists but the Catholics were the first Christians. At least the first ones worth noting. For centuries they were the only Christians besides little heresies here and there (no I'm not counting the Eastern Church because they're a Thing). Lutherans and others are just The Little Heresies That Could.
This would be so screwed up to make as a romance manga. There'd have to be murders and such. Several dead beliefs are out there.
How would you handle the Arians, Cathars, and countless other Christian heresies? Catholicism having lots of kids and then murdering them? @_@
EDIT: Well, Arianism is old enough that I'd imagine you could make it a younger sister, or even a twin.