After reading back over that comment, I noticed some things that I failed to clarify, which I felt were rather important, if not vital, to the point I was trying to make... so I added another reply:
I wouldn't call religion a mental illness per se, however I would say that it causes mental failure.
Religion requires a compartmentalization of thought to enable one to simultaneously believe drastically different or contradictory things. For instance, a man or woman in their daily lives will generally require just as much proof of an amazing claim as any other person would. But when it comes to their religious beliefs… even the most outlandish claims, which are in direct violation of the physical laws of the daily world we live in… and are wholly self-contradictory etc… they take these without question.
This type of cognitive suppression and compartmentalization, which enables them to essentially overcome cognitive dissonance, has numerous ill effects. Its symptoms include such things as a complete inability to even entertain that any notion that in any way conflicts with their religious belief might have any validity. It causes them to oversimplify complex issues that might conflict with their beliefs if studied too deeply. It causes them to treat people of other beliefs as sadly wrong individuals… or to do such things as treat homosexuals as abominations who are going to burn in hell… while usually at the same time being polite to their faces. They don't understand that them telling a person that they're perverse and evil and abominations could have negative repercussions in that persons life. They believe that God hates gays, so they feel 100% right in sharing that view. They believe that the bible teaches certain things, and therefor they have a right to legislate those particular morals onto everyone else, even when others might not share their personal beliefs.
This type of detachment from reality… and from critical thinking… the oversimplification of issues, the outright willful ignorance of matters that might threaten their beliefs… these things do harm to society. When you have people that willfully make themselves ignorant of scientific facts and medical and scientific progress etc… and practice rote memorization of religious dogma and use it to try to fight scientific progress and hold humanity back as a whole while violating the basic rights of other individuals in society, while being largely painfully ignorant of the facts and complexities of the issues at hand as a result of the very belief system they are trying to maintain… this, as a whole, has a negative effect on society at large.
People unaware of sexual education because of the forced ignorance of the subject, leading to STD transmission and early pregnancies etc… or discrimination against gays because "God says so" rather than any real world reasoning… or teaching bigotry against people of other cultures if they don't share the same beliefs etc, because any other belief must be wrong if it's not Christianity, and other cultures entail different beliefs on non-religious ideas as well… but being different is also seen as dangerous to the conservative status quo in the cultural sense… so you have a general misunderstanding of people like Muslims etc… and rather than addressing the real foundations of the problems with Radical Islam today, you have such ignorant mindsets as that all Muslims are towel headed lunatics who hate life so much that they just want to kill us while they end their own miserable lives out of depression, hatred and jealousy of our Freedom… when this is entirely wrong! This leads us into wars where hundreds of thousands of lives are lost and whole countries descend into turmoil.
People fail to understand that the problems we have with terrorism today are generally the direct result of Religious Fundamentalism… and while this terminology is bandied about as the latest buzzwords… people utterly fail to grasp it's implications. The problem with Religious Fundamentalism is merely in people actually doing what their religion teaches them, even commands them to do! The only reason the rest of the religious aren't following suit is because of a combination of things… failure to understand their own religion fully, and a failure to obey it's teachings. This along with the scientific and cultural secular advances of the past 2,000 years have led to a society, especially here in the U.S., where a majority of people claim to be religious and will steadfastly and vehemently defend their beliefs, but generally have a very poor knowledge of those beliefs and a very poor concept of how those beliefs interact with and even contradict their own daily lives, beliefs and actions. This being another result of the aforementioned compartmentalization of thought that enables them to function from day to day while maintaining a belief system that contradicts the world around them, and the knowledge that enables them to function in it. (dying is bad. if you walk off this bridge, gravity will make you fall and die. certain diseases will kill you. the dead cannot come back to life. etc. bob cannot be 2 places at once. your biological mother cannot be 2 different people. praying will not pay your bills.)
I have seen many times over the results of this compartmentalized mental disconnection from reality… and while many apologists would like us to believe that religion is a good thing, they fail to understand that the kind of "good religion" they're referring to is nothing more than a rather large failure of actually being religious, and mostly following the secular wisdom of the past few thousand years in violation of and in contradiction with the religions they claim to wholeheartedly believe in and steadfastly support. And they do this while denouncing the people on both ends of the spectrum simultaneously… those who are against their beliefs, or do not believe them at all…. and those who share their beliefs, but believe them 100% and obey them in accordance with that belief.
How hard is it to understand that there is a problem when the only way your beliefs are a good thing is when you don't really believe them or follow them fully? And how can you claim their infallibility while at the same time saying that some of them are not relevant or don't need to be followed? (hint: the compartmentalization I talked about earlier). How can you say that Gods word is infallible and must be obeyed, but then ignore all the things he absolutely commands you to do? How can you claim someone is evil or wrong and justify retribution toward or denigration of such people, when you yourself invalidate the very words you claim as your authority? compartmentalization of thought… willful blindness of contradictions… willful ignorance of complex situations… refusal to even acknowledge any error in logic relating to your beliefs.
I could go on and on about this… but I'm not the best writer by any means, and I would hope that my explanation so far has shed a little light on this.
(The closing comment about Jefferson refers to the following quote from Thomas Jefferson, taken from the Notes on the state of Virginia (1781-1785): "The legitimate powers of government extend to such acts only as are injurious to others. But it does me no injury for my neighbor to say there are twenty gods or no God. It neither picks my pocket nor breaks my leg.", which was made in the context of Separation of Church and State.)
I suppose that I should clarify something...
When I stated that "This type of cognitive suppression and compartmentalization, which enables them to essentially overcome cognitive dissonance, has numerous ill effects.", I should have clarified that the real danger with this is the fact that this same set of mental self-induced disabilities and dysfunctions follows through into their daily life... so that when faced with real life cognitive dissonance, they resolve it internally in the same manner. They ignore contradictions, they willfully maintain ignorance of ideas and knowledge that might contradict their preconceptions... and while these are all traits we somewhat share... they are also much more likely to simply completely fail to see even the most obvious contradictions, or to be able to use any real level of critical thinking skills or logical reasoning ability etc.
As these self-induced shortcomings spill over into their daily lives, this is when the damage really starts being done. At this point it is no longer simply a harmless fantasy that affects nobody but themselves, at this point they are failing to understand the world around them and using their own self-induced ignorance of concepts and it's resulting painfully flawed understanding of the world as motivations to force their beliefs onto other people by passing laws against them, or supporting wars against them, or simply spreading hate about them, while completely failing to see what they're doing for what it really is. They see what they're doing as perfectly right and perfectly good as extensions of God's perfect will.
If these types of people really did just have a fantasy that harmed no one but themselves, I wouldn't really care... I'd have little problem letting them run around and spend their lives as simpletons. I'd just pity them a bit and ignore them. But when we have them directing national policy and law because of their ignorance en masse... and when my life is personally detrimentally effected by their disassociation with reality and inability to think rationally etc... I have a problem with it.
Although Jefferson made his comment about it neither picking his pocket or breaking his leg... I think even Jefferson would understand that egregious violations of personal freedom in the name of religion is just as heinous... if not even more so.