Score one for the arc of the moral universe. It’s long, but it bends toward justice.

I’ve been thinking about this metaphor since Tuesday afternoon, when a Minnesota jury found police officer Derek Chauvin guilty of murdering George Floyd last May. The arc-of-the-universe quote is usually attributed to Martin Luther King Jr., who borrowed it from the 19th-century abolitionist minister Theodore Parker. But it was popularized by Barack Obama, who adored it so much that he had it woven into a rug in the Oval Office of the White House.

And I adore it too because it reminds us of something essential about America. We are a nation that was born in both slavery and freedom, in oppression and liberty, in hatred and in love. Our story is the ongoing struggle to rescue and preserve the better angels of our nature — as Abraham Lincoln called them — from the darkness in our souls.

That’s the small-l liberal dream — to make good on the ideals in our founding documents, no matter how many times we might flout or forget them. But liberalism is under fire these days from two very different sides, who only agree on one thing: Our country is fundamentally broken and cannot be redeemed. It must be destroyed and made over as something else.

That was the heart of the Trump revolution, which wrapped itself in the American flag and sullied America at the same time. Over and over, Donald Trump told his followers that their institutions were corrupt and “rigged,” Mr. Trump’s favorite adjective. Elections, armies, courts, laws — they are base, crooked and dishonest. If you don’t like what they do, ignore them or throw them out.

But that was hugely dishonest in its own right. Our institutions are not perfect, of course. But over time — and with constant struggle — we have found ways to improve them and bring them into closer accord with our collective creed: liberty and justice for all.

In 1955, two white men murdered a 14-year-old African American boy, Emmet Till. Acquitted by an all-white jury, they bragged about their evil deed to a magazine reporter afterward. And on Tuesday, a multiracial jury convicted a white police officer of murdering a Black man.

If that isn’t moral progress toward our national ideal, I don’t know what is.

But don’t tell that to illiberals on the Left, who have mounted their own challenge to the arc-of-the-universe motto in recent years. There is nothing good or just about America and its institutions, they say. Indeed, “America” is a problem to be overcome, not a promise to be loved.

Witness the now-ritual demands to take down memorials and rename schools dedicated to Washington, Jefferson and Lincoln. They did horribly racist things, including enslaving human beings and defending the same. So they must go as we remake the country.

Around what? The illiberals have no answer, at least not one that can move a people or a nation. King, Frederick Douglass and other warriors for justice in America understood the power of our founding ideals. We could never get closer to those ideals unless we also believed in them, and in the capacity of an imperfect America to create a more perfect one.

Or consider another mantra of the illiberal Left: “defund the police.” The term means different things to different people, of course. But at its heart, like all attacks on liberalism, it reflects a basic cynicism about our institutions and ourselves.

The police are racist. The police are a legacy of slave patrols. The police are an occupying army. The police do not protect us. Tell that to the heroic Capitol police officer Eugene Goodman, an African American man, who likely saved Mitt Romney and other legislators from injury or death on Jan. 6. Or tell it to the current and former members of the Minneapolis Police Department, who took the stand to testify against Derek Chauvin.

Of course, one guilty verdict does not change a country. Racism is a central fact of our national story, and it remains an enormous problem in our criminal justice system. But we will not make it better unless we believe in the righteousness and integrity of America and — especially — of its original creed.

The arc of the moral universe is long, but in America it bends toward justice.

Believe it, now and forever. And don’t let any American tell you otherwise.

Jonathan Zimmerman (jlzimm@aol.com) teaches education and history at the University of Pennsylvania. He is the co-author (with Signe Wilkinson) of “Free Speech And Why You Should Give a Damn,” which was published this month by City of Light Press.