They think they are telling the truth based on facts

It always amazes me when people think their version of the truth is the only possible truth (after extensive research? from reliable? sources finding the "real" truth? truth that THEY are already predisposed to agree with).
... Even worse when they think they MUST explain it to you because you "just don't understand" and you need to be educated by them. Grrrrrr.
... In their defense though, they're not really lying, ... they really believe in their facts whether their facts are true or not.