“Where is sacrifice in our lives today?” he asked.

Audience members shouted answers without raising their hands. Mr. Lau-Lavie, a rabbinical student, walked into the crowd to share the microphone. What act in today’s society, he asked, was painful enough, messy enough, to approximate ancient sacrifice? Finally he offered an answer: unplugging from the Internet for one full day a week. It would hurt, sure, but it could also be cleansing, he said. Then he confessed: “Giving up digital for 24 hours is so healthy, but I don’t do it, because I’m addicted.”

The gathering was the monthly Sabbath service of Lab/Shul, an experimental pop-up synagogue that is still in what Mr. Lau-Lavie calls its beta phase. Mr. Lau-Lavie was gearing up for Purim this weekend and the confluence of Passover and Easter in April, when the congregation will mix traditions in a Last Seder.

“One of the ways to describe what this is about is creating sanctuary,” Mr. Lau-Lavie told the roughly 60 people who came to City Winery for the more-than-two-hour service, Lab/Shul’s fifth. Some were non-Jews or atheists; some were observant men and women comfortable in skullcaps. The conversational style and claim to counterculture, the texts and videos projected on screens, the emphasis on arts and music, resembled nothing so much as a modern evangelical Christian church. Mr. Lau-Lavie invited congregants into a big tent: “It’s a god-optional, bring-your-own-god, do-it-yourself-god, everybody-friendly community.”

These are precarious times for non-Orthodox synagogues in New York. According to a 2013 survey by the Pew Research Religion and Life Project, fewer than one-third of American Jews belong to a synagogue, and barely one-quarter say religion is very important in their lives, compared with 56 percent of the general public. The share of Jews living in a household where anyone belonged to a synagogue fell to 39 percent. In a 2001 survey, it was 46 percent. The decline has been especially acute in Reform and Conservative congregations, many of which have closed or merged to stay afloat, even as the Orthodox community expands.

The wreckage, in turn, has created opportunities to improvise.

“We’re in a veritable explosion of experimentation,” said Rabbi Elie Kaunfer, executive director of Mechon Hadar, a nonprofit group that teaches and supports Jews in communities of learning, prayer and service. “It used to be that there were three or four major flavors of Jewish life, and you belonged to one of them. Now you see things grow up in the spaces between those more institutional expressions of Jewish life, and they’re really taking off.”