Uprooting believers.

[Tonus]

I think this guy is on the money when it comes to the process of deconversion (it's a series of videos):

He also does a very nice video summary of the breakdown of the different sources of the OT in Karen Armstrong's A History of God.

As for the OP: Seth McRaney's book You Are Not So Smart does a good job of covering a number of areas of research on how the human mind works. One thing to keep in mind is how our inherent biases can take a subconscious belief and reinforce it against virtually all attempts at reason. A good example are phobias- an irrational fear that we know is irrational, yet we seem helpless to confront. So we make excuses for it, even though we know that the excuses don't explain the phobia.

A person who is terrified of spiders will tell you that they "hate them" because they're creepy, or they're disgusting, or they're poisonous, and so on. It's true that some species are poisonous, but the phobic person is scared witless even by those that they know are harmless. Many people do find them creepy or disgusting, but they don't exhibit the kind of debilitating terror that the phobic person does. Those excuses are simply attempts at dealing with a reaction that the person knows is not rational, yet that he cannot control.

Now, imagine a person has subconsciously accepted that god exists, and apply those same mechanisms to that belief. Presto! A belief that is unshakable because you won't let anyone even try to test it, and which you buttress with every conceivable bias and heuristic that you can pull out of your ear. This is why, for many people, deconversion is a very traumatic experience even when the end result is beneficial.