(RNS) — I have done my share of traveling in Europe, and when I am there, I visit cathedrals.

Most are majestic, and they are filled with Christian art that would take a decent docent a decade to unpack for me.

I have never been to Brussels, though I would like to visit. And when I am there, I expect to make a special trip to the Cathedral of St. Michael and St. Gudula. That is the subject of Flora Cassen’s new book, “Stained Glass: A Reflective History of Antisemitism.”

The cathedral is, by all accounts, a masterpiece. Built between the 13th and 15th centuries, it rises above the old town on its own little hill, and when the lights hit the stonework at night, it looks like lace carved out of sky.

But I imagine myself stepping inside. I would look intensely and intentionally at the stained-glass windows — the ones donated by Belgium’s first two kings in the 19th century. And inside that beautiful space, an erudite guide might tell a story about a Jew who, in 1370, was accused of torturing Communion wafers. 

It is an expression of one of the libels that tormented Jews during the Middle Ages — one of the most bizarre — the host desecration libel. It resulted in six Jews burned at the stake and the rest expelled from the city.

And there it is. In the windows. In the tapestries. In the chapel. In the capital of the European Union. Today. 

Flora’s book is itself a modern medieval tapestry — of Jewish and European history and family memoir, the story of a 15th-century Jewish woman named Beatrice de Luna — also known as Dona Gracia — and the story of Flora’s own grandmother, Pola, who fled the Nazi occupation of Belgium through the Congo. 

So, why does this book matter, and why do you need to read it?

It is because of what you already know. Antisemitism is rising — on university campuses, in social media feeds, even in food co-ops in Brooklyn.

We have fought back. We have invested enormous resources in Holocaust memory — museums, memorials and curricula.

Why hasn’t it worked? Why, after everything, are Jews in Europe quietly researching what it would take to leave? Why are my colleagues on university campuses afraid to wear a Star of David? Why does the hatred keep coming back, wearing slightly different clothes?

Because, despite the Dylan song, the times really aren’t a-changing.

In 1913, the modern Yiddish and Hebrew poet Zalman Shneour wrote a poem — “Again the Dark Ages Draw Nigh.” He was writing about the Mendel Beilis blood libel case in Russia: 

Again, the Dark Ages draw nigh!

Do you hearken, O man, do you sense it —

The whirling and swirling of dust and the sulphurous scent in the distance?

Yes, the Dark Ages are approaching. Every medieval anti-Jewish libel still exists. The blood libel accused Jews of murdering Christians. There were accusations that the Jews poisoned the wells, which brought upon the bubonic plague and accusations that Jews controlled the weather. 

Those libels have not disappeared. The blood libel survives in Nicholas Kristof’s New York Times piece claiming Israelis used dogs for rape and the equally obscene accusation that Israel harvests organs from the dead. During COVID-19, conspiracy theorists suggested that Jews or Israel created or spread the virus for profit or global control. There was the insane charge that Jewish lasers from space caused the California wildfires.

That is why this book is so important. We have not left medieval history. Or, rather, medieval history has not left us.

But there is more. As Flora says in the podcast, the Jew is the test of whether a nation can contain diversity. That is why what starts with the Jews never ends with the Jews. 

As David Frum put it recently in The Atlantic:

A society that turns on its Jewish minority eventually devours itself. The shots aimed at the windows of synagogues are aimed at larger targets. The advent of liberal modernity was announced by the dismantling of ghetto walls. The re-erection of those walls sounds a note of doom, and not only for the Jews.

Want to evaluate the mental health of a society?

Then observe how it treats its Jews.

In which case, many societies are flunking.