Fundamentalist Christians spend their entire lives trying to fit a square peg in a round hole. They believe in one "truth" and try to make every aspect of their lives fit that belief. Over time inconsistencies between their belief system and their observations multiply, so they learn to ignore the inner child that asks questions out of curiosity. Eventually the worldview of fundamentalist Christians becomes nothing more than faith and opinions rather than facts and evidence. Even worse, they fight personal and legal battles to convince others to act the same way, and increasingly their target is students in public schools.
As evidence mounts for natural rather than supernatural explanations, fundamentalist Christians in the U.S. have begun to feel threatened. In response they have attempted to change the evidence. Early attempts by "Creationists" to undermine the scientific theories of evolution and an "old" earth failed to meet the high standards of scientific evidence and sometimes amounted to fraud. Also, legal arguments for teaching creationism in public schools failed in the courtroom.
Fundamentalist Christians responded by rebranding creationism as "Intelligent Design," but again their attempts to replace the teaching of evolution with Intelligent Design in public schools failed with the Dover, PA decision. Increasingly frustrated by the use of secular reasoning to reject religious arguments, fundamentalist Christians started to argue against the legal concept of Separation of Church and State that has been upheld for centuries in U.S. courtrooms. In their revisionist accounts of U.S. history they maintain that the Founding Fathers meant for the U.S. to be a "Christian Nation." As a result they advocate for a theocracy to replace our current federal government. Imagine replacing the world's leading model of democracy, perhaps the most successful government in history, with theocracies like those in the middle East, where society has regressed to the Middle Ages.
All of these fundamentalist Christian advocacy efforts take a great deal of time and money. Many spend their lives trying to reverse the tide of secularism they feel awash in. They ban books and boycott corporations and media outlets that don't promote their worldview. They shelter their children from anything they consider profane by pulling them from sexual education classes, and in some cases forbidding them to socialize with anyone not in their church community, in essence making that community a cult.
Fundamentalist Christians were taught their beliefs at an early age, and they want these beliefs to be passed on to their children in school. They also recognize that the task would be easier if students were not shown any contradictory evidence. Thus, teachers in fundamentalist Christian schools teach their students only intelligent design but not evolution, religion but not science, and revisionist histories such as "Christian Nation" that are based on ideology rather than historical evidence. Furthermore, they discourage students from "questioning authority," which robs students of the opportunity to develop critical thinking skills. Withholding evidence and discouraging students from thinking independently amounts to brainwashing. Furthermore, without developing skills such as independent, objective, critical thinking, fundamentalist Christian students will never contribute to the creation of new knowledge; they will simply parrot the religious dogma they have been spoon-fed their entire lives. Students should be taught how to think, not what to think.
And it's not enough for evangelical Christians to indoctrinate their own children. They want every student, including those in public schools, to be taught religious doctrine rather than the tools of science such as logic. Currently there is a movement in Texas that is pressuring textbook publishers to include biblical references to figures such as Moses, even though there is no evidence of Moses' existence outside the bible. And since Texas is one of the most populous states in the U.S., such changes would likely propagate into the textbooks purchased by schools throughout the U.S.. To extend this failed teaching paradigm to all children in the U.S. would be a disaster not only for us but for the entire world, as the U.S. is still the world leader in production of scientific knowledge, which has raised the standard of living for the millions of people who live in the developed world.
In summary, the fundamentalist Christian movement is an obstacle to scientific and social progress. Scientists and educators have to spend increasing amounts of time and money countering the inaccurate claims of fundamentalist Christians. They need help from the public, but the public first needs to recognize the threats posed by evangelical fundamentalist Christianity. Freedom of speech and of religion are at stake.