Intellectual freedom is one of humanity’s greatest gifts—and biggest burdens. Our ability to ask questions, to test ideas, to doubt is what separates us from our fellow animals. But doubt can be as terrifying as it is liberating. And it’s the terror of doubt that fosters the toxic, life-negating cult of creationism.

That fear is on full display throughout HBO’s new documentary Questioning Darwin, which features a series of intimate interviews with biblical fundamentalists. Creationism, the documentary reveals, isn’t a harmless, compartmentalized fantasy. It’s a suffocating, oppressive worldview through which believers must interpret reality—and its primary target is children. For creationists, intellectual inquiry is a sin, and anyone who dares to doubt the wisdom of their doctrine invites eternal damnation. That’s the perverse brilliance of creationism, the key to its self-perpetuation: First it locks kids in the dungeon of ignorance and dogmatic fundamentalism. Then it throws away the key.

And that dungeon is much darker than most Americans realize. The creationists interviewed in Questioning Darwin—including their abominable doyen, Ken Ham, a wily businessman who is already fundraising off his ill-conceived recent debate with Bill Nye—returned again and again to the same depressing subjects. Death, suffering, pain, sorrow, disease: These, creationists inform us, are what await any skeptic, anyone who questions the word of God. Pastor Joe Coffey neatly sums up their objections to natural selection:

If all we are is a product of this random mutation process, then where does morality come from? Where does hope come from? Where does love come from? Where does anything that makes us a human being really come from?

The answer, to creationists, is simple: There is no love, no humanity, in a world with evolution. Humans must have been designed by God; if we weren’t, then we’re mere animals, lacking in morality and dignity, consigned to a pitiful and pointless life of struggle and dolor. Evolution, one true believer informs us, is “incompatible with biblical Christianity” because it recognizes the permanence of death and leaves no room for a second coming. Creationists are consumed by repressing the existential panic that often attends acceptance of reality. Instead of confronting that terror, they’ve retreated into an elaborate fantasy.

So deep is their delusion, in fact, that many creationists are perfectly willing to acknowledge their abandonment of reality—on camera. “Truth is not an assimilation of information,” insists one fundamentalist in Questioning Darwin, explaining why no amount of evidence could change his mind about human evolution. “There’s one truth, and that’s found in the Bible.” A creationist pastor takes this illogic to its harrowing extreme, freely conceding that he would perform endless mental gymnastics to justify the seemingly unjustifiable conclusions of biblical text.

“If somewhere in the Bible I were to find a passage that says two plus two equals five,” the pastor states plainly, “I wouldn’t question what I’m reading in the Bible. I would believe it—accept it as true and then do my best to work it out and to understand it.”

This ideology might seem fairly benign. And it’s true that, by itself, creationism damages only those who choose to believe it. But here lies the true peril of the dogma: No creationist is content to keep her beliefs to herself. Creationists don’t merely proselytize; they brainwash their own children and push their creed into public schools across the country. Creationists teach their children not only that evolution is evil, but that studying evolution, even thinking about it, is a sin that leads the soul to eternal damnation.

“What [Darwin] has done is worse than murder,” proclaims one creationist in the documentary. “All the death and suffering we see here isn’t the result of a creator God,” says another. “It’s sin”—such as the acceptance of evolution. (He doesn’t explain how so much death occurred before the publication of The Origin of Species in 1859.) Creationists spend countless hours telling their children that scientists and biology teachers are spouting the devil’s lies. Creationism is a mental prison with no hope of release, for the only escape hatch—intellectual inquiry—has been sealed off by years of suppression.

The conflict between creationism and evolution is, to many believers, an all-out war between God’s word and humans’ sins. That’s why parents are so determined to prepare their children for battle. One mother in Questioning Darwin home-schools her children to prepare them to defend their creationist beliefs against secular “attacks.” We see pastors casting an ominous eye over their congregations, warning them of the horrors of the Darwinian worldview and the hellfire that awaits those who are lured into its trap. Creationists discuss evolution with a combination of abject fear and muffled rage, ranting that natural selection sounds “crazy” while calmly asserting that Adam and Eve shared Eden with vegetarian dinosaurs. They blame Darwin for Hitler (a time-honored smear) as well as for drug use, murder, and an endless parade of horribles. Question the Bible, creationists tell their children, and you will soon be drowning in barbarity.

This view isn’t benign or wacky: It’s poisonous and medieval. Creationists reject not just evolution but most of the Enlightenment and pretty much all intellectual development since. Rather than celebrate the brilliance of the human mind, they disparage free thought as dangerous and sinful. Instead of extolling the virtues of creativity and imagination, they malign all unorthodox ideas as immoral and wicked. For all creationists’ insistence that evolution denigrates humanity, creationism is fundamentally anti-human, commanding us to spurn our own logic and cognition in favor of absurd sophism derived from a 3,000-year-old text. It turns our greatest ability—to reason—into our greatest enemy. Using our brains, according to creationism, will lead us to sin; only mindless piety can keep us on the track to salvation.

It’s easy to scoff at all this, to giggle at the vivid weirdness of young Earth creationism and then shrug it off as an isolated cult. But the 40 percent of Americans who reject evolution, as well as the tens of thousands of children or more who are being brainwashed with it in publicly funded classrooms, aren’t laughing. Creationism is built to metastasize; those who believe it won’t rest until everyone else believes it, too. True believers yearn for the rest of us to be locked up in the same mental prison where they have consigned themselves and their children. They insist that evolution has robbed us of our humanity. But in reality, it’s their twisted gospel that aims to strip us of the very thing that makes us human.