NPCs don't join, they follow. That's an important distinction. They're not going to "understand" and just decide to become outcasts. They're NPCs, making sense doesn't factor into it.
You can destroy the ideas they follow with ridicule and shame and confusion. You want them to think it's wrong to continue with those topics because that's not the thing anymore.
To attract people to follow truth, you still need to treat it as propaganda. Being true makes the job easier, but since you're up against propaganda that seeks to destroy it, you still have to present yours with more skill and reach than your opponent.
Probably the biggest thing is being able to think like people who aren't you. Not easy for most people.