It is a psychological term called cognitive dissonance... Basically, somebody becomes so adhered to a political or religious ideology that they INTENTIONALLY REFUSE to believe actual facts and shun and attack those who try to prove facts to them. For example, somebody who joined a cult that believes that beach sand is actually the only form of sustenance the human body needs will attack anyone who tries to tell them that if they do not eat normal food and drink normal fluids, they will die. They will force themselves to eat beach sand, until they are dead, either from starvation or from the abrasive effects of the sand in their internal organs.
And what makes this phenomenon more terrifying is that even people we consider normal and intelligent can and will succumb to this if they become too passionate about a political, social, or religious movement. Many of the people who quit their high paying jobs and went with Jim Jones to their doomed one way trip to Jonestown had been successful medical doctors, teachers, scientists, and engineers. A part of their minds KNEW that what they are doing will certainly lead them to their horrific deaths, but they went anyway.
ETA: The flat earth movement is another perfect example of this phenomenon. A KiwiFarms forums member who is a good friend of mine had told me how he had a long time buddy from high school who was very smart, athletic, self confident and extremely successful academically and socially. They drifted apart and he assumed this guy went to become a marine biologist like he had always wanted to do. When they met again years later, my friend is just utterly bewildered to find that this guy had COMPLETELY succumbed to the flat earth cult. Every fucking thing that came out of his mouth was about disc worlds, conspiracies, and how every fucking thing in the sky are projections onto a dome and shit like that.
View attachment 7558196 Needless to say my friend is NOT friends with this weirdo anymore...