Cognitive Dissonance

The thing about ‘cognitive dissonance’ is that once you believe something—I mean REALLY believe it—you tend to modify your perception of reality to fit the belief, rather than the other way around. 
Properly managed, it’s a great tool for shaping and locking in beliefs within a cult. Start with a small lie, then build up to the whoppers. 

Another thing about cognitive dissonance is that it’s easy to detect in people with whom you disagree – just hard to tell whether it’s their dissonance or your own.