The answer is simple — there were no other churches for 15 centuries before the Reformation other than the RCC.
There are the Eastern Orthodox, who were already in schism by that point.
But how without being circular do you know for sure that your church is the true church when Eastern Orthodox people say they are the true church?
As already mentioned, the Eastern Orthodox still technically have true churches with Apostolic Succession, sacraments, etc. There’s a schism that exists primarily due to our disagreements over the Papacy, but that doesn’t diminish the fact that they contain most of what we do. (Note: Historically, there’s been a lot more political and theological reasons behind the division, but the Papacy seems to be the main cause now.)
However, I believe Catholics have the same problem in the sense that instead of having to only interpret a infallible Bible, they also have to interpret what an “infallible” church or religious authority says.
The difference between the Bible and the Magisterium is that the Magisterium can address confusion. All Scriptures contained within were written some time before 100 A.D., and any confusion is left up to the interpreter. For the Magisterium, if there’s suitable confusion, it can respond with definitive declarations.
Furthermore, the Magisterium’s teachings tend to be clearer to a modern audience. Much of it is contained in the
Catechism of the Catholic Church, which might be dry but is also very accessible for a modern day Catholic to understand the fullness of Catholic teaching. After all, it’s based around the last 2000 years of needing to clarify the teaching, as opposed to just 100 years of doing so, and is written for a modern audience, not a first century audience.
Also the argument about 33,000 Protestant denominations reminds me of an atheist who asked me “Out of all he thousands of religions how can you ever know which one’s the right one.”
It’s less a problem with “which one is true” and more the problem with alleging guidance from Someone who values unity. The Holy Spirit both values unity and was promised to lead us into all truth (John 16:13). Neither are exemplified in Protestantism, which shows a fundamental problem with its foundation.
My question is if one seems to contradict the other wouldn’t that prove that the foundation of Roman Catholicism would be destroyed.
Well, we don’t have a contradiction, and such a contradiction would seem to show Jesus as a fraud. As a result, I’m not sure the question is worth dwelling on at this point.