Michael V. Hayden, a retired four-star Air Force general and former director of the federal Central Intelligence and National Security agencies, said something truly profound this week.

“It’s really hard to unseat a belief with facts that wasn’t based on facts to begin with,” he said in an April 30 CNN television interview with anchor Don Lemon Monday night.

Ponder that a moment.

It perfectly reflects religious thinking and political thinking during this Trumpian interlude in American politics, which, most unfortunately, is having an enormously corrosive, destructive effect on American life.

Both phenomena are “post-truth,” as if such a realm actually exists. Even if people stop acknowledging it, truth remains. But in religion and cults of personality like Trump’s, the faithful simply forego facts for emotional transcendence. They arbitrarily choose to believe in invisible beings and wholly unapparent traits of character because it somehow feels better than facing reality.

The momentary challenge is that the leader of the free world is using his rabid popularity with his disgruntled, disenfranchised base not to neutralize America’s enemies but to make war on us — on truth and our essential institutions of democracy. His goal is clearly an authoritarian takeover of the machinery of government, where truth becomes only what he says it is.

Hayden’s new book, The Assault on Intelligence: American National Security in an Age of Lies, is a laser-focused rebuttal — a blaring alarm, really — to this all-out frontal attack on the core value of the post-medieval Enlightenment: the absolute necessity of fact and reason (not “received truth,” baseless dogma) in determining truth.

When Hayden led American forces in Europe at the height of the Bosnian civil war in 1994, he said, he concluded that “the veneer of civilization … is quite thin.” Sarajevo, ground zero of the war — a previously “cultured, tolerant, vibrant place,” Hayden wrote in a recent op-ed — had been “ripped asunder” in a collision of Muslim Bosniaks against Christian Serbs and Croats.

“I wondered what manner of man could pick up a sniper rifle and shoot former neighbors lining up for scarce water at a shuttered brewery,” he wrote.

The answer: a man not thinking rationally. Hayden lamented:

“Over the years I had learned that the traditions and institutions that protect us from living Hobbesian ‘solitary, poor, nasty, brutish and short’ lives are inherently fragile and demand careful tending. In America today, they are under serious stress.”

President Trump and his cultural bomb-throwers reject the Enlightenment ethos that values “experience and expertise, the centrality of fact, humility in the face of complexity, the need for study and a respect for ideas,” Hayden wrote. He said it’s inevitable that “post-truth” was Oxford Dictionaries’ word of the year in 2016, reflecting an erosion of the primacy of facts against emotion and unsubstantiated private belief.

This erosion imperils everyone. As it did in Nazi Germany on the eve of World War II. As it did during the Crusades. And the Vietnam War. And the McCarthy era. Quoting historian Timothy Snyder in On Tyranny, Hayden wrote:

“To abandon facts is to abandon freedom. If nothing is true, then no one can criticize power because there is no basis upon which to do so.”

“Post-truth,” Snyder added, “is pre-fascism.”

Welcome to America’s cowardly new, pre-fascist world.

And truth, it turns out, can be quantified. In his first 400 days in office, the president made more than 2,400 false or misleading statements, according to the Washington Post. Nonetheless, his approval rating among Republicans is more than 80 percent. How can we square this circle, reconcile these divergent facts? In the Post piece, Daniel A. Effron, an associate professor of organizational behavior at London Business School, wrote:

“Some supporters no doubt believe many of the falsehoods. Others may recognize the claims as falsehoods but tolerate them as a side effect of an off-the-cuff rhetorical style they admire. Or perhaps they have become desensitized to the dishonesty by the sheer volume of it.”

All bad, no doubt. Possibly worse, according to Effron, Trump insiders use another disingenuous strategy to give credence to their leader’s scatter-gun lies: “They encourage people to reflect on how the falsehoods could have been true.”

Effron says his research suggests that this strategy can be effective because it leads susceptible people to conclude that it’s not unethical for political leaders to lie even when everyone knows they’re lying.

For example, Effron noted, when the president retweeted a video seeming — falsely — to show a Muslim immigrant assaulting an American, White House Press Secretary Sara Huckabee Sanders, rebuffed a journalist questioning the clip’s authenticity. “Whether it’s a real video,” she thinly proposed, “the threat is real.”

So there you have it. Truth isn’t fungible, the same everywhere, like oil, but infinitely elastic instead.

Except it isn’t.

We should urgently heed Michael Hayden’s and Daniel Effron’s chilling words. Truth is under sustained, purposeful attack by the president and his “neo-fascist” supporters. Heretofore, in trying to defend against it, too many of us have been our own worst enemies by being criminally gullible or just going along to go along.

With both religion and Trump, we are only able to see the truth when we consciously ignore the sirens of deception — “beliefs not based on facts” — hidden in our own DNA. Otherwise, “post-truth” wins.

Image: Bits and Splits, Adobe Stock, standard license