I am writing this post to try explain the concept of cognitive dissonance and how it applies to religions in a simple way. The reason behind this is that often we hear the term cognitive dissonance thrown around, but do we really understand it.
They best way to understand cognitive dissonance is by placing yourself in a situation.
So here is the situation: Let assume you pack bananas onto a truck for 8 hours a day, 5 days a week. Now this may not seem a fruitful job to many people, however the truck into which you pack the bananas has a logo which reads "Bananas feeding the poor and needy". Seeing this logo everyday eventually starts making you feel important in your work as you are helping the poor and the needy. You stay at this job for 10 years and on the first day of the 11th year as you are going to work a guy stops you and says "you know those bananas in that truck with the logo, "Bananas feeding the poor and needy", get dumped at a waste dump everyday."
Will you believe this person. Simple answer is no. If the person then shows you video evidence you will most probably start doubting your jobs worth, but may still not be convinced. This is cognitive dissonance. What is happening is that your brain refuses to believe evidence that is getting presented to it as for the last 10 years it has been told differently.
So in a nutshell, you have been told something that nullifies everything you have done up to that point. Your brain is now in a state of not been able to handle what information it has been told. So the brain, being the wonderful organ it is, refuses to accept this new fact. This makes it feel that everything it has invested so far still has worth. After all a basic human desire is to feel important! You can see this just by saying thank you to someone for something simple and seeing how their face lights up.
So how does this apply to religion?
When a theist gets exposed to new truths that contradict what they believe. They will shut down and make up excuses as to why they do not agree with you. The simple reason for this is that they have invested so much time in religion at that point that it is impossible for their brain to discard religion.
What makes it even worse in religion is that if a theist then goes back to a church the ministers/rabbis/mullahs there will tell them that this new information is a lie. This makes the brain feel good again and it denies the new truths even more.
For more on Cognitive dissonance see the links below.