If you’re actually asking how to reach people about a delusional belief they hold it seems often if you try to show them evidence that directly contradicts their belief, argue reasonably, etc they will often become more entrenched in a defiant ‘you can’t teach me anything’ stance to protect their perceived self identity from unwarranted attacks. You have to demonstrate the warrant first.

A possibly more effective method seems to be temporarily accepting their proposed perspective, and asking skeptical questions to reduce their confidence in the delusion. Providing the questions to work it out themselves instead of forcing the conclusion. I haven’t read it yet but I’ve seen https://en.wikipedia.org/wiki/Combating_Cult_Mind_Control recommended as a study on cults and breaking their indoctrination & control tactics but I’m open to more contemporary recommendations if anyone has them.