So why has the news that a synagogue in the Netherlands stopped posting the time of services upset me above all? Because it is vivid proof that anti-Semitism is driving Jews underground in the West.

For some time now, many kippah-wearing Jews have adopted the habit of wearing baseball caps when visiting Europe. Young people think twice before wearing Israeli-flag T-shirts when they wander the streets of Paris. Or before carrying a backpack with the name of their Jewish youth group prominently displayed. A number of years ago, I met a Jewish woman from Brussels who told me that she had asked her teenage children not to wear their Jewish-star necklaces in public. She acknowledged that she was embarrassed to have asked them and relieved when they agreed.

During a trip to Berlin, a friend gave me directions to an out-of-the-way synagogue. After some intricate explanations, he added that if I got lost, I should look for police on the street with submachine guns. “That,” he noted, “would be the entrance to the synagogue.” But I should also keep watch for men in baseball caps and follow them. “They will lead you to the synagogue.” I did get lost, and followed some men in baseball caps as instructed. I was relieved when I saw the police. I had found it.

For many years, Jews have known that when visiting a European synagogue, they must bring their passport with them and expect to be interrogated by guards outside the door. I now call ahead to let a synagogue know that I am coming. And that does not always guarantee entry. A few years ago, I was turned away from a synagogue in Rome.

Jews have been living defensively for a long time. But when a synagogue, as a precaution, decides not to post the time of services, we have reached a new level. In Spain in the 15th century, many Jews sought safety from persecution by converting to Christianity but secretly maintained their Jewish practices. They lit Shabbat candles in inner reaches of their homes where no one from the street could see, and eschewed eating pork or shellfish. They became what Spaniards called Marranos, a term of degradation comparable in some ways to kike. Some Jews who converted did not maintain the traditions. This did not, of course, guarantee their safety when the Church, state authorities, and the mob began to seek out Marranos for persecution.

I use the term, however reluctantly, because it captures what I am seeing today. Most Jewish students on American campuses have not been subjected to overt acts of discrimination or verbal abuse. But many among them feel they have something to lose if they openly identify as Jews. If they are active in Hillel, the Jewish student organization, they may be informally barred from being active in progressive causes—for example, racial and LGBTQ equality, climate-change mitigation, and the fight against sexual assault. Those who want to be elected to student government are learning to scrub their résumés clean of any overtly Jewish or pro-Israel activities. They are not abandoning their Jewish identity; they are hiding it. They have become Marranos.