This is tough because it's less a matter of tactics, but awareness. Reading about Jews radicalized to martyrdom by the inquisition doesn't exactly give you a hands-on toolkit to address the issue of fundamentalism in society today.

It does, however, serve as a reminder that folks who dig their heels in when presented with a new development that contradicts the familiar - myself included - are going through a shared, relatable process and the end result of that is always something new. It's inherently going to be its own kind of advancement or progress, even if the intention is preservation of the status quo. That leads to thoughts like, "What does this person want to preserve?", "What new development could come out of this and is it useful to consider?", and "What does this person fear this new idea will disrupt?" Those are handy thoughts to keep in mind when trying to reach a common understanding of the change at hand.

"The fundamentalist mind" isn't an alien, foreign reaction held by a few disillusioned radicals, but a totally normal and relatable coping mechanism (albeit taken to a horrific extreme in some rare cases). That serves as a basis to talk about it constructively, IMO, whether I'm dealing with a new client who wants to change everything (but without changing anything, naturally) or my uncle who proudly shares misinformation from Facebook that supports his comfort with "the way things used to be." This includes questioning my own motivations when I catch myself jumping through hoops to make my old beliefs or practices work in a new context.