The hate and ignorance espoused by many of religious leaders should not have a place in this modern world. Unfortunately, many people in all parts of the world, will blindly accept whatever they are told without sufficient thought or critical analysis. Followers of any religion should critically think about whether the teachings of their leader feel morally right and will result in positive actions. If their teachings will harm others or do not feel right, then they should be discarded.
If one has ever taken the time to listen to a fundamentalist speak, such as a YEC, you can come to the blatant conclusion that they're mind is a product of cultural conditioning. They're unable to think for themselves, and every word of their Holy Text is, by default, always correct; disregarding contrary evidence. Some assert that science is the manipulation of the Devil, and that anyone who embraces science could spend eternity in Hell.
They are deluded, lunatic, psychological slaves in complete denial of reality and the natural world. It's terrifying.
Religious fundamentalism by definition rejects any facts that are contrary to its own dogma. This is a definition of irrational thought which is a manifestation of a mental disorder. Then there is the little fact of invisible and imaginary friends. If you are an adult asserting you have an invisible friend who created the universe -- you have lost your marbles... Period. If this invisible friend talks to you and you hear the voices in your head, you probably need medical intervention. If you are a fundamentalist who promotes these ideas but don't actually believe it, then you are a habitual liar. In either case you have a mental disorder...
Religious fundamentalism by its definition rules out all facts not contrary to its own dogma. This is a perfect example of irrational and illogical thought. Then there is the little matter of invisible and imaginary friends. If you are an adult asserting that you have an invisible and imaginary friend who created the universe, you have lost your marbles in every sense of the word... Period. If this friend talks to you and you hear the voices in your head, your mental disorder is serious enough to warrant medical intervention. Sadly, 50% of our population describe themselves as such, hence the reason we sweep it under the carpet and deny it exist.
Humans are not born with hate and intolerance; this is something that is taught to them by uneducated, ignorant, intolerant individuals or organizations. It's a form of brain-washing. Accompanying this "learned" way of thinking is the risk that those who believe it is their personal mission to change or convert others to an acceptable way of life may end up causing emotional, psychological and / or even physical harm to others. The most extreme example would be the Nazi party's influence on the citizens of Germany and other European nations.
Most of the religions of the world were created hundreds of years ago. Being fundamentalist, therefore, is like being fanatically stuck with some older ideas. Of course, older ideas are not bad always, but some of them must be revised. Do religious fundamentalists ever revise the older ideas? They don't. Sometimes, even when situation demands, people demand, they don't change their interpretations. For example, Taliban shoots Malala Yousefzai and takes pride in doing this. This is abnormal mentality, a kind of compulsive obsessive disorder.
While many fundamental followers are indeed sick people, that does not mean all fundamentalist are sick. We have seen many leaders in the fundamentalist church preach against homosexuals, then get caught red handed engaging in all sorts of homosexual acts. I believe the opportunist see the weak minded, uneducated, and mentally ill as a source of easy living. They prey on these groups for donations while living a life of lies.
It's silly to think that people with extreme views on religion are automatically mentally ill. Do we also assume all right wing Republicans are mentally ill? How about those who swing to the far left? A person can have a strong, passionate, unwavering opinion on any topic and still be completely mentally stable.
While I may lack a clinical background, it is my belief that subscribers of fundamentalist religions are not suffering from mental illness, but rather an unfortunate interpretation of societal pressures.
This is not to say they're delusional or mentally ill, but rather to say that their worldview is, to them, a logical response to the events they see unfolding. While fundamentalists may not respond in the most constructive ways, their belief or faith grants them security and purpose in a world where many of us probably seek other forms of solace and escapism. While a delusion may be a permanent mental illness, fundamentalists can be reasoned with and changed (barring accompanying mental illness, of course). While the mentally ill may be more prone to fundamentalist teachings, this does not necessarily mean all fundamentalists are legitimately ill. Some may just suffer from isolation and a lack of ideas.
Essentially, I think that fundamentalist religion is mistaken and fear filled, but I don't think it always is a sign of mental illness. I think people who follow this way are not very intelligent because they need to take things literally and do not go deeper. They are afraid to hear other interpretations.
Fundamentalism isn't a manifestation of mental illness, but it is a natural outlet for people who have mental illnesses. They can easily get caught up in the beliefs and the more extreme they go the more they are encouraged. But there are people who have sincere beliefs that are fundamental and who do not have mental illnesses.