What's so difficult? You have one God. Only He's not really just One God, but Three. You have the Pope, who leads the Church. This is a person, who is elected by people (the College of Cardinals), and is subject to all the frailties of being a person. Except when he (and it is a he by convention) is speaking
ex cathedra, in which case he's completely and absolutely correct in a manner that means he cannot possibly be or become incorrect. And this was implemented retroactively from a century ago, so not only did you have all of these people from the Second Laterean on, but all the Popes in the past retroactively became infallible. Except the ones who weren't really Popes, because they were anti-Popes. Oh, and he's only infallible if a handful of very specific conditions come true just right and he's talking about the proper subjects. And then you have Communion, when you eat bread and wine, which symbolically represents the Body and Blood of Christ. Except, when you actuallly eat it, it isn't just symbolic, but it literally becomes the Body and Blood of Christ. Except for to the people who say it doesn't "literally become," strictly speaking, but the spirit of the Body and Blood still literally arrives directly and exists alongside that of the bread and blood...err, wine.
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Um...well...at least they aren't Calvinists?
