My annual custom on the last day of the High Holy Days is to daven at the Yom Kippur minyan of Chabad of Rego Park. Not a chasid, not a member of the Chabad-Lubavitch chasidic movement, I feel great spiritual authenticity in the atmosphere of intimacy, surrounded by a few hundred other worshippers, which Rabbi Eli Blokh creates.

His Yom Kippur services take place in the basement social hall of the Queens Jewish Center, a large Modern Orthodox synagogue around the corner from my apartment.

Most of the seats are empty at the start of services, both at night and in the morning, but soon fill up, mostly with elderly men and women, separated by a wooden lattice mechitza, from the former Soviet Union. Rabbi Blokh, born in Moscow, conducts a city-wide outreach to fellow Jews from the FSU; his announcements during services alternate between Russian and English; Russian-language machzorim take their place each year on a table in the back of the hall with English-Hebrew High Holy Days prayerbooks.

Many of the émigré worshippers at the Chabad services show up only once a year, on Yom Kippur, in time for the Yizkor memorial service.

Rabbi Blokh commented on this phenomenon before Kol Nidrei last week.

He didn’t berate the irregular worshippers. He didn’t criticize. He didn’t encourage them to come more often. Instead, he made himself – the whole congregation, the entire Jewish people – part of their ranks.

“We are all Yom Kippur Jews,” he declared. That means, he said, every Jew has a spark of holiness that is lit on the Day of Atonement.

He came to praise the people whom many rabbis usually condemn, the so-called once-a-year Jews. There are no once-a-year Jews, Rabbi Blokh said; we all express our Judaism in different ways, according to our own calendars.

Strange, I thought to myself, that Yom Kippur, the most-demanding of all Jewish holidays, the day when eating and drinking and other taken-for-granted acts are banned, maintains its grip on Jews who have a tenuous connection to many Jewish traditions. Why aren’t there “Purim Jews”?

I found the answer that morning, in a break during shacharit, in “Future Tense,” the 2009 book by Rabbi Jonathan Sacks, chief rabbi of England, my choice of reading material that week. In his essays on the existential Jewish condition, he explains an anomaly: Why does Yom Kippur continue to have such resonance, even for people who were raised in societies where atheism reigned and the practice of – or education in – Judaism was illegal and dangerous while other less-demanding holidays have less attraction?

“The more demanding the task, the greater commitment it evokes,” Rabbi Sacks writes. “When people made Judaism easy, they found that their children preferred other ways of life.” The Rabbi cites Passover; most Jews keep some part of that tom tov, despite its endless list of requirements; few Jews observe Shavuot, which requires almost nothing.

The demands of Pesach determine its value, Rabbi Sacks writes; it’s popular because it’s not easy.

Shver tzu zein a Yid. It’s hard to be a Jew. That Yiddish expression usually tells why Jews leave Judaism. Last Yom Kippur, on the hardest day of the Jewish year, it also told why all the seats of the Chabad of Rego Park service were filled.