Where would you draw the line between someone advocating for their belief vs shoving it down your throat? Does the "extremeness" of the belief factor into the judgement? If so, how are we not back at the original question of the definition of what an extreme belief is?

Why are you implying that advocating for your belief and making someone else endure that advocation is some kind of right? It is not.

As to your question, there are a lot of extreme beliefs that sometimes end up becoming true. IIRC the person who first advocated for washing hands because of tiny "things" that could make people sick, he was laughed out of the room, and he was right. It was an extreme belief at the time.

I basically reject the whole premise that "no belief is extreme" because there are countless examples throughout history of extreme beliefs that the world considered extreme, and they turned out to be true. Didn't make the belief any less extreme at the time.