Show where Christ established the Roman Catholic Church (you may also want to discuss the concept of Christ's first church with apostolic Protestant groups and the Eastern Orthodox Church). Christ established his church, full of human beings who would differ -- some honestly, some not -- on certain matters.
But I reject the premise implicit in your post. The Catholic Church is not a church that has existed, unchanged, since Peter's time. That's nonsense. In fact, the RCC became so thoroughly corrupt at certain times in its history that people of good faith had to leave it.
The Church is where Christ is, and in some times and some places that has not been the RCC.
As I said, give some proof. You are the one who challenges it, it is up to you to show your side. You just want to say what’s right and what’s wrong without offering any proof whatsoever.