Pardon my ignorance but if I may ask, how does one differentiate Catholicism from other sects of Christianity for the purposes is this tournament? As someone raised religious but not catholic specifically I do not quite understand the lines and want to know before submitting characters.

My friend... I did so many google search about the eastern orthodox church and the oriental orthodox church and omg. It wouldn't be catholism without fighting haha

The rule was in communion with Rome meaning from a sec that could receive the eucharist at a Roman Catholic Church. However, I think it's a bit too complicated since it's hard to know if Sophie from Mama Mia is Greek Orthodox or Greek Byzantine Catholic. When there are too many rules it's not that fun (East–West Schism </3) I'm just going to say that if the blorbo is orthodox they're allowed.

If your character is/implied to be Christian (and not canonically another religion) and from the Middle East, Ethiopia, Eastern Europe, or India(or a fictional country that resembles these countries), then it's fine.

If a character is sometimes protestant and something catholic, I think Steve Rogers has been written as both, then that's fine. I want to try to avoid actual Protestants though. So no Rue from Euphoria or Sheldon's mom from The Big Bang Theory or Ned Flanders from the Simpson. If you're not sure, I would use geographical, time period and blorbo's ethnicity for context clues. White Southern Lady with the last name Jones? Probably Protestant. Some dude from 1940s Chicago with the last name Kowalski. Probably Catholic.  

TLDR: if their church has stain glass or bones and their priest wears a fancy dress, then go for it!