Here's a good discussion semi-related to your question.

[religionnews.com]

Fear of having to deal with someone with a superior intellectual capacity for observing and synthesizing the evidence presented.

An innate fear of the facts...

We scare people out of complacency and comfort of believing in an afterlife.

because people don't like being told they are wrong. if a theist told you that you that your beliefs were wrong and had indisputable proof, would you likely believe it or would you stick to what you have always known? people like comfort and security in their ideals. it is very hard to change somebody's ideas through sheer force alone.

I think they know deep inside it's a fantasy world and they defy internal logic with defiance and passionately take sides with the believer side of the planet

Religious people aren't satisfied unless everyone is drinking the kool aid. Deep down, they know their beliefs are unjustified. When someone comes along who doesn't share those beliefs, their defenses immediately come out. Usually the best defense (or most common) is to go on the offense. The tribalist instincts kick in and it becomes an "us vs. them" scenario.

I try to find common ground with people, so they have trouble putting me in the "them" camp. I think it's best to promote understanding, rather than to play into their game of going back and forth attacking each other. I like to keep things peaceful, but I'm not above punching someone if they get out of line.

Have you ever watched that guy doing street epistemology? Anthony {long Italian last name}? He had an excellent approach when taking to believers that I use all the time with great success. Basically just asking leading questions and letting them arrive at the conclusions. It keeps the hostility down and plants seeds of doubt that may take hold over time.

Years ago a lady friend told me when I professed my lack of belief I attacked every thing religious that she had been taught and believed in since she was old enough to remember.

She hated the fact that I was a historian and often stated to her that there were no legitimate primary sources to support "her god" over "their" god or any god, only heavily edited text altered to fit the era that they supposedly originated.