I have seen the question posed on Progressive Christian blogs and on numerous atheist/humanist/agnostic websites, “Why is it impossible to debate a fundamentalist?” Truth Wins Out, a website devoted to ending LGBT prejudice, posits that the reason it is impossible to debate with fundamentalists is because fundamentalists tend to cherry-pick verses out of the Bible to make their points, instead of taking into consideration the historical and cultural context surrounding said “clobber” verses. Exploring Our Matrix, the blog of biblical scholar Dr. James F. McGrath, suggests that it's impossible to debate fundamentalists because of their cognitive dissonance. God Of Evolution, a website dedicated to reconciling Christianity and evolution, posted the thoughts of one commenter who made the argument that you cannot debate creationists in particular, because creationists/fundamentalists are not critical thinkers wanting to debate for the purpose of gaining new knowledge, but instead are debating from a fixed-position based on self righteousness and stubbornness.
I suspect all of these reasons are correct, and even fit together, but I would like to raise a different— and I feel more simple— answer. The reason why it is impossible to debate a fundamentalist is because their very language psychologically traps them into their frames of mind. Allow me to explain. Each person's journey into fundamentalism is different, but the process of injecting fundamentalist thought into the minds of newcomers and solidifying that school of thought is the same. When somebody is new to a fundamentalist church, they hear phrases in sermons like “God's word”, “God's way”, “the world”, “compromise”, etc.
When one hears these phrases associated with certain concepts, the next time they hear about these concepts outside of their fundamentalist environment, they will automatically associate those concepts with the phrases used to describe them in the sermon. In this way fundamentalist rhetoric acts as a prison guard to the mind, ensuring that freedom is never achieved. Below are examples of fundamentalist vocabulary and their definitions, followed by an example of what I've just said above.
My interpretation of the Bible = “God's word”
My political views = “God's way”
Believers in evolution = “atheists”
Rejection of literalism = “compromise”
Secularists, liberals, libertarians... anyone not Republican = “persecutors”, “assailants of Christianity”
Gender equality = “attacking the institution of the family”
Our small individual sect = “the church”
Another common phrase is “the world”— a very vague formless phrase meant to refer to anyone outside of a fundamentalist's respective church, whom he/she believes is conspiring to persecute them, and at which, they can project all of their condemnation.
When a newcomer in a fundamentalist church hears the preacher say, “So what's it gonna be? God's word or atheism in public schools? Christians cannot compromise while the world continues to persecute the church and attack our values!”, the new fundamentalist automatically associates “God's word” as the Bible, evolution as “atheism”, “the world” as anyone who is outside of his sect, and “the church” as being narrowly defined as his one specific religious group (all others, literally, be damned). This way when evolution is brought up, say, in a parent-teacher conference, the “prison guard” that is fundamentalist vocabulary kicks in and activates our newcomer to “take a stand” against “the world” who, he is told, is trying to persecute him.
That's why it is impossible to debate with a fundamentalist. By replacing “my” with “God” and melding beliefs about authority with authority itself, fundamentalist vocabulary has left no room for humility, reason, openness, doubt or change, and it is a mental wall many of them will not climb over. Ever. Their very vocabulary traps their mind and keeps them imprisoned within their own fanaticism.
By labeling anyone who disagrees with them as “atheists”, “compromisers” and “not true Christians”, fundamentalists leaders depend upon the simpleminded to pack their pews and attend their events. This model of faith is inherently irrational, and therefore tends to drive away thoughtful people who have perfectly reasonable doubts, while rewarding people who either lack the concern or the curiosity to question their beliefs by making them feel like that lack is really “strong faith".
But there is hope. I feel like the younger generations are calling out religious conservatism for what it is: bullshit. This is not to say that older people of previous generations aren't equally as skeptical of fundamentalism, but undoubtedly millennials are kicking against fundamentalism like never before.
On a personal note, the passive-aggressive language of fundamentalists used to get under my skin and sometimes still does. When someone close to me ordered me a creationist book from Answers In Genesis two months ago, I told them I wasn't interested. Their response— in sing-song voice— was “Okayyyy... just want you to go to heaven.” Since then I've learned to pity fundamentalists. I pity them because they don't detect the words that wound others within their vocabulary. Nor would they care even if they could detect them. I pity them because they are unaware of just how brainwashed they are. In short, again, I agree with those who say there is no debating a fundamentalist. You can only pity them, and stand back and watch from a distance as they tear up the world that is immediately around them.