All you need to know is that Catholicism is the one true faith and that all the other so-called Christians are heathens that will burn in hell for all eternity.If you look at this Wiki page, there are literally hundreds of Christian denominations.
Why? I just don't get it. Because last I checked they all claimed to believe in the same God.

Actually, I find this propensity for religions to break into innumerable sects to be one of the strongest arguments against the whole concept. They can't all be right, yet they all have members that know they are following the true line of faith. So either the gods are assholes that mess with people into believing different falsehoods, or this feeling of knowing one follows the one true faith has something to do with human tribalism that has been present throughout our history as a species. As a rationalist, I think it's the latter.