". What's the difference between someone who believes in nonsense and someone who has a "sincere" belief in nonsense?"

Your question is phrased so that no answer is possible. The relevant question is "How do different people process and act according to their beliefs in what WE are sure is nonsense?"

You add "...we must never forget that religious extremism and fundamentalism are very dangerous forces in our world...

Yes indeed... **some** do horrible things in the name of religious fundamental extremism. Many do not.
The difference, it seems, between you & I... and several others here... is that I take into account the thousands of years of human history as humans tried to cope with the mysteries of "life, the universe and everything" and quite naturally created stories & myths trying to explain it all. I factor in my awareness that much of the evidence and reasoning debunking those stories & myths is only a couple of hundred years old, and the stories are rich, complex and part of the very fabric of most of humanity! You tell ME how you'd go about 'educating' hundreds of millions, most of whom have little or no access to the data & reasoning that 'we educated ones' use to move beyond the stories & superstitions!

I HOPE that someday, reason will be be taught formally in schools all over the world and that gradually, superstition will be superseded by reason... but in the meantime, all I can really do is argue and talk... and VOTE for legislators who will curtail the influence of fundamentalist religion in the laws and culture of my country!

Right now... if I had a magic button that would suddenly 'convince' all those holding deep religious beliefs that they had been wrong all their lives... I would not push the button. The chaos would be much worse than just gritting my teeth and taking one-more-little-step!

short form of my rambling: Yelling at the ignorant and insulting them never does diddly-squat!


Remember Lucy in the "Peanuts" comic strip?

Old Peanuts cartoon:

Lucy, talking to Linus: "Change your mind!"
Linus just looks at her.
Lucy.."CHANGE YOUR MIND!!
Linus looks more intimidated...
Lucy.."CHANGE YOUR MIND, I SAY!!"

Lucy, walking away, disgruntled and mumbling."Boy, it's hard to get people to change their minds these day!"