The U.S. grew from a “backwoods country” to one of “greatest nations the world has ever known” thanks to science — but that pillar of America is eroding, astrophysicist Neil deGrasse Tyson warns.

Science deniers “rising to power” now create a “recipe for the complete dismantling of our informed democracy,” Tyson says in a powerful new video that he promises contains “what may be the most important words I have ever spoken.”

“People have lost the ability to judge what is true and what is not, what is reliable, what is not reliable,” he says in the above video, which he posted to Facebook Wednesday. “That’s not the country I remember growing up in. I don’t remember any other time where people were standing in denial of what science was.”

Many scientists have become frustrated with the Trump administration’s anti-science moves, which has led to them holding the Earth Day March for Science protest Saturday.

Recognize what science is, and allow to be what it can and should be: In the service of civilization.

Now, in his new video that’s interspersed with news reports, he warns of the threats that climate change skeptics and those against vaccinations pose.

One of the clips is of Vice President Mike Pence when he was a congressman enthusiastically rejecting the existence of evolution. “Let us demand that educators around America teach evolution not as fact, but as theory,” Pence says.

Tyson praises science as an “exercise in finding what is true” that’s based on peer-reviewed experimentation backed by other experiments and counter-experiments that gives birth to an “emergent truth.”

He points out that science is “not something to toy with.” “You can’t say, ‘I chose not to believe in E=mc2,’” he says, referring to physicist Albert Einstein’s corroborated theory of special relativity. “You don’t have that option. It is true, whether or not you believe in it.”

“The sooner you understand that, the faster we can get on with the political conversations about how to solve the problems that face us,” he adds.

And Tyson warns that every minute someone is in denial of a scientific truth delays the “political solution that should have been established years ago.”

“Recognize what science is, and allow to be what it can and should be: In the service of civilization,” he says. “It’s in our hands.”

Watch the powerful video in full above.