Protestantism Is a Deal Breaker

Protestantism is a very large target, so to speak. Even if you can't prove god exists, doesn't mean you should be an atheist who denies the existence of god. You just be agnostic on the question. For me, catholicism has an authority structure, scripture, tradition, magisterium. Protestantism has 66 books inspired in errant, infallible rule of faith. My question is, why should i believe that that is true? Where is the evidence for that? So the deal breaker for me is that i can't get to that.