GRONINGEN, Netherlands — Every other week, as the Jewish holiday of Shabbat approaches in the Dutch city of Groningen, another, more mundane ritual takes place.

A group of self-appointed volunteers begin calling around and texting friends on WhatsApp.

They are confirming the attendance of fellow members of the Jewish community at that week’s Saturday morning service.

Amid growing fears of violent anti-Semitism following several attacks on Jewish institutions across Europe, the Groningen synagogue — like many others — now operates behind fortified defenses, and in a nearly covert fashion.

The community has an emergency plan in case of an attack, and local police keep watch on site during services. It doesn’t publish details of its services, and relies on worshippers to keep each other informed of the schedule and muster up attendance in private chat groups and over the phone. Members are trained to vet visitors at the entrance.

"The layer that was holding back anti-Semitism from growing has been breached" — David Gurov, student

For an outsider, the congregation has become easy to overlook — and that’s, partly, the point.

Alec Farber, 18, who moved to Groningen to study from New York, missed a number of holy days before he realized a congregation existed. He finally gained entry to a service after making friends with other Jewish students. He had to bring identification, and flag his arrival in advance.

"It was actively difficult," Farber said of the process. Back in the United States, he said, "my synagogue posts every service on its website.”

For many, the difficult logistics are a small price to pay to keep the congregation safe from the type of violent attacks that have devastated other Jewish communities in recent years.

Given the family ties and connections between communities in different countries, the effects of an attack reverberate across Europe. Members of the Groningen congregation knew people caught up in the 2015 shooting at the Great Synagogue of Copenhagen, and in the recent attack in Halle in Germany on Yom Kippur, in which a 27-year-old far-right assailant live-streamed himself besieging a synagogue, killing two people.

The incident in Halle is a turning point, warned David Gurov, a 20-year-old student originally from Cologne, whose friend was barricaded in the German synagogue as the gunman tried to force his way in.

"History is getting forgotten,” he said. “And the layer that was holding back anti-Semitism from growing has been breached."

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The Jewish history of Groningen could hardly be a more poignant warning of how bad things can get.

Groningen's domed synagogue was built in 1906 in the center of what was then a thriving Jewish quarter, with kosher butchers and a bakery famous for its challah bread. When Nazi Germany invaded the Netherlands in 1940, some 2,800 Groningers were registered as Jewish. The community was deported almost in its entirety to concentration camps by 1943.

A handful of people survived the camps. A few more escaped deportation by chance, or by finding somewhere to hide. By the end of the war, a congregation that used to fill the synagogue with 1,000 worshipers on High Holy Days mustered a few dozen people at most. It was typical for survivors to have lost over 100 family members each.

"It's like putting a bomb in a family. One survives, another does not," said Johan van Gelder, a descendant of survivors, and a journalist who has written several books documenting the destruction. "They were innocent. It was inhuman. It was a disgrace to humanity."

Many survivors found Groningen to be too full of ghosts and chose to emigrate to start new lives elsewhere. The road that housed the rabbi's residence, once known as the “street of music” for its renowned musical families, emptied out and became a red-light district. The synagogue was sold off and became a laundry, its stained-glass windows smashed out for steam pipes.

In the 1970s, the synagogue was saved from demolition by a single vote cast in a city council ballot. A campaign led by a young Jewish woman from Amsterdam, Lenny Wolgen, rescued it for restoration and reopened in 1981. The Torah scroll, thought to have survived the occupation hidden in a bank vault, was restored to the head of the synagogue.

Now, eight decades since it was almost entirely wiped out in the Holocaust, the Jewish community of Groningen is focused on protecting what’s been salvaged in the face of rising anti-Semitism.

In a recent survey of 12 European Union countries by the European Union Agency for Fundamental Rights, a large majority of Jewish people said they feel anti-Semitism has increased over the past five years and are cautious about wearing anything that could identify their faith in a public place. Over a quarter of respondents to the survey in the Netherlands said they had experienced anti-Semitic harassment in the past year.

"I'm concerned about anti-Semitism becoming normalized, and slipping into the mainstream" — Tom Burghard, 20

In Groningen, many are also wary of an increased normalization of hate speech, made more bitter by an unwillingness by non-Jewish friends to recognize its seriousness.

"I'm concerned about anti-Semitism becoming normalized, and slipping into the mainstream," said Tom Burghard, 20, who said he noticed an increasing sense of distance among young people from the Holocaust his grandparents lived through.

"If it's not a physical attack, people don't take it seriously,” he said. “They don't understand this is how it starts."

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The secrecy and added security measures can hamper the community's ability to thrive.

On the Jewish New Year Rosh Hashanah, the community had too few men to make up a minyan, a quorum of 10 required in traditional Judaism to hold public worship and read from the Torah. The gathered crowd went home disappointed.

But each ceremony held at the Groningen synagogue is a celebration of resilience, members say. Children play freely at the feet of the men as they worship in their draped prayer shawls, while women chat and watch from the sidelines.

Many of the holy days remember triumph over past persecution. Ceremonies conclude with the l'chaim toast to life, before the congregation moves to a cozier room for warm discussion over coffee and cakes.

"They say after a massacre it takes 200 years for people not to be afraid anymore. I'm not going to be around for 200 years" — Judith Klop, worshiper at the Groningen synagogue

When services are not being held, the main entrance and half of the hall of the synagogue is opened as a secular event space that hosts photography exhibitions and concerts.

Opening the space up to the general public can carry risks. During one such event — a remembrance for the deportation and killing of the residents of a Jewish home for the elderly in the 1940s — a member of the public entered and became argumentative, blaming Jewish people for various ills.

The incident put some in the congregation on edge. Members of the community described feeling constantly alert, aware that their gatherings could be a target. Others said they had left fear behind.

"They say after a massacre it takes 200 years for people not to be afraid anymore. I'm not going to be around for 200 years," said Judith Klop, who was attending a service with her two young adult sons. "I decided not to be afraid."

The Jewish community of Groningen is not expecting the past to repeat itself in so lethal a fashion. But its members stressed that many in the Netherlands are too oblivious to the dangers that discrimination and an increasingly polarized political climate pose not just to Jewish people, but to all minorities.

Burghard said he saw echoes of the past in current discrimination against Muslims. "Saying that they have a secret plan, that they want to impose Shariah. If you can convince a certain number of people this group is a threat, then you get into a situation where violence gets justified."

The Netherlands has still not adequately acknowledged that the slaughter of the Jewish people was not undertaken by Nazi invaders alone, but required the passivity, and in some cases collaboration, of Dutch people, according to Van Gelder, the journalist and historian.

"There still is the danger that it could happen again," he said. “Probably not in the same way as it happened in World War II, but there is still a chance of a new genocide. It can be Jews, it can be Muslims, it can be Christians. The danger is always there.”

As he spoke, he leafed through black and white photographs of the citizens of Groningen who were sent to the camps, and never returned.

"What can I say about it. You can't get them back. We have to go on with our lives," he said sadly. "Let us be a warning for the future. That's the only thing we can conclude from the whole thing."