Oh I hate that term, the Church never uses the term Roman Catholic so neither will I except to say that I don’t like it. Now that we’ve gotten that out of the way, I think a good place to start is with history. Why did the Eastern churches accept the authority of the Pope until 1054? Why is there no evidence whatsoever of first amongst equals or other such things until after the great schism? Let’s go to the Protestant Reformation, why is it that Luther and Calvin thought themselves as self-appointed prophets, Calvin especially. Here’s the problem with questions like that, they make the assumption that the Church has to be corrupt, and while there is corruption in the Church that does not mean that corruption is in the Church’s dogmas, Muslims and Mormons claim the same thing, that doesn’t make them right. Like you said though, it’s a hypothetical question, but I always refer back to history when I have this discussion. My major stumbling block with Mormonism, is the claim that men can become gods, my major stumbling block with Islam, is the claim that Jesus was not crucified, in spite of overwhelming historical evidence. Since, even from history, we know the Catholic Church is the original Church, you can find this in secular history books, we either have to concede that Jesus was a liar, or the Church’s Dogma is not corrupt. That being said, there are things in other groups that are true, most Protestants could be considered trinitarian, so that’s correct, Muslims, claim to worship and serve the God of Abraham, so there’s that. I’m always skeptical of any group that tells you that they’re a restoration, whether it’s Islam, Seventh-day Adventist, Jehovah’s Witnesses, Mormonism, Etc.