I think it helps to understand that everyone experiences doubt no matter how zealous they appear. Your words may not be listened to in the moment, but they'll be heard. Sometimes it takes years but every scratch you put into the monolith hastens its eventual crumble.
Besides, when your alternative is to strengthen it through adversity, you literally have nothing to lose. Not even that self-righteous satisfaction of telling them off is real - you're just filling a role built into the conflict by the person you should be mad at. It's counter-productive.
Hard to keep resilience in the hopes that what they hear will be listened to eventually, but I guess you're right.
Sidenote on this, watch Red State. JTHipster, can I get a backup on this? It's among the most severely underrated films in recent years and features some of the best lead role acting I've seen in a long, long time-- but anyway it relates to the cult psychology that we're talking about here, worth an hour and a half.