Her grandparents — “cultural” Jews on one side of the family, more traditional on the other — came to the United States from the Old Country a century ago and didn’t change their level of religious observance (or non-observance) during their lives. Their spiritual lives were a straight line.

Her parents, newly married, moved to Westchester County in the early 1970s, joined a Reform temple and never left it. Their spiritual lives were also a straight line.

Abby Sher’s spiritual life is a curve.

Raised in her family’s Reform congregation, where she became bat mitzvah, Sher, now 37 and a resident of Brooklyn’s Park Slope neighborhood, would attend worship services sometimes during college and afterwards, sometimes not. “Usually Reform or Conservative,” she says. One of those denominations will probably be her choice when she and her husband join a synagogue soon, but, she says, “I’m open to any denomination that speaks to me.”

Over the years, she has taken to devoting chunks of her time — sometimes more, sometimes less — to personal prayer for her friends’ and her own needs.

An author and performer, Sher identifies as “Reform — Reform in progress,” but adds, “it changes every day. I’ve been a drifter,” theologically that is, Sher says.

When it comes to Jews her age, Sher is not alone. She and young Jews like her are an increasingly typical segment of American Jewry on the cusp of 2011.

Sher, unlike her parents’ or grandparents’ generation, but like members of her under-40 generation, is in theological flux, likely to change her Jewish identity, her formal affiliation and her level of Jewish observance or practice. According to statistical and anecdotal evidence, many American Jews, especially in their 20s and 30s, wear different Jewish hats until they find one — or, as is often the case, several — that feel comfortable.

Call it Generation F — for fluid.

The term post-denominationalism has been tossed around for probably a decade now, and Jewish institutions have been grappling for some time with how to respond to the trend, how to reach people like Abby Sher. But the new fluidity — fueled by new thinking about Jewish identity and the pervasive influence of the Internet — suggests a deepening of the trend, so that today the nature of community itself is undergoing a vast transformation. It’s yet another sign of the Open Source Judaism movement, which fosters the creation of cyberseders and individualized prayer books that afford greater personal control over one’s expression of Judaism.

“It’s a long-term trend, and it’s not just a Jewish phenomenon. It’s a general phenomenon,” says sociologist Steven M. Cohen, director of the Berman Jewish Policy Archive @ New York University’s Robert F. Wagner Graduate School of Public Service.

Representatives of several Jewish institutions agree.

“This past decade has seen a new trend: a marked shift away from sub-labels to a disinclination to categorize at all,” Rabbi Elie Kaunfer writes in “Empowered Judaism: What Independent Minyanim Can Teach Us about Building Vibrant Jewish Communities” (Jewish Lights Publishing, 2010). “No broad label works to identify people who are true to their own complex Jewish journey.”

The national movement to establish independent prayer groups, which are not formally tied to any branch of Judaism, a movement that Rabbi Kaunfer has spearheaded, has proven particularly attractive to Jews in their 20 and 30s. “A world without convenient categories is a world that calls on people to take more ownership of the type of Judaism they want to practice,” he writes.

New ways of thinking about Jewish identity are feeding the trend.

“There is a significant trend for emerging adults to break identity assumptions in an age where Jewish identity has become multiple and fractured,” says Rabbi Shmuly Yanklowitz, a founder of the Uri L’Tzedek Orthodox social justice movement who now serves as senior Jewish educator at the University of California at Los Angeles’ Hillel chapter.

Rabbi Daniel Brenner, who this month moves from his position as executive director of Birthright Israel NEXT to director of Initiatives for Boys and Men at the Philadelphia-based Moving Traditions organization, finds that “the fluidity in American Jewish life exists in a context of general decline in ideological commitment to religious and ethnically defined movements.

“The fluidity of young Jews in their thinking around movements is a product of multiple factors,” he says, including “the rise of Chabad,” the chasidic sect which offers a nonjudgmental, no-demands alternative to standard synagogue behavior, and “the democratizing impact of the Web.”

Young Jews are marrying later and starting families later, so they can push off the time to join a congregation for their children’s Jewish education. Some point to the Hillel experience in college — many chapters sponsor several prayer groups under the same roof at the same time as an attractive pluralistic model for post-college life. And there’s Birthright Israel, which introduces the Jewish state and intensive Jewish life to young American Jews in a non-denominational setting.

Others point to the differences between their formative years and their parents’ or grandparents’. The earlier generations, products of the Depression here or immigration from the former Soviet Union, sought a spiritual anchor; when they felt at home in a particular shul or denomination, they were unlikely to leave it.

Their children, raised in relative affluence, feel less tied to a single institution or form of Jewish expression; similarly, most have lived in several homes and worked in several jobs in the years after they finish college.

“It’s easier to move around. It’s more acceptable to experiment with what makes you comfortable,” says 24-year-old Rachel Cahn, who works as a program associate for Birthright Israel NEXT.

“I can be comfortable in a variety of [Jewish] settings,” says 27-year-old Jake Wilkenfeld-Mongillo, a Connecticut native who was raised Conservative but now calls himself “traditional egalitarian.” A communications associate for the Jewish environmental group Hazon, he is a member of an unaffiliated congregation in Park Slope, Brooklyn.

The denominations themselves are, to varying degrees, redefining themselves, embracing practices that represent a break from their collective past or from the Jewish lives lived by many of their members. The rabbinical association of the Reform movement recently published the first Reform guide to the Jewish dietary laws, stressing the kashrut laws’ ethical importance for congregants who largely do not follow them. The Conservative movement’s Jewish Theological Seminary is putting an increased emphasis on keeping the mitzvot, underlining for a self-defined halachic movement the value of the commandments that many members do not keep. Orthodox institutions, seeing a decrease in the percentage of Orthodox Jews in this country from about 15 percent a generation ago to less than 10 percent today, according to most surveys, have turned to the Internet, social media and coffee house-type events to attract disaffected Jews.

“Among liberal Jews … the boundaries are especially porous,” Central Synagogue’s Rabbi Peter Rubinstein said in his Rosh HaShanah sermon this year. “Synagogues no longer necessarily hire rabbis, cantors or educators trained at the denominational seminary of their movement. Small congregations are merging across denominational lines. Congregants are not choosing synagogues by denominational ideology.”

This phenomenon also characterizes the Orthodox community outside of the New York area, which tends to be “far less observant … much more fluid,” says Steven Bayme, director of the Contemporary Jewish Life Department at the American Jewish Committee.

The key today is choice — options for a Jewish lifestyle are available today in this country that weren’t in earlier years.

There’s less “brand loyalty” to established synagogues or denominations, especially during the recent recession, observers in the Jewish community say.

The change, they say, is evolutionary not revolutionary, but this year saw a continued increase in the number of independent, unaffiliated minyans across the U.S., an expansion in the Synagogue 3000 Next Dor initiative that brings Jewish programming to young Jews in non-synagogue settings, and the release of a Pew Forum on Religion & Public Life survey that reported that about 40 percent of Americans of all ages had either dropped their religious affiliation or switched to another affiliation.

In Jewish terms, this usually means changing to another denomination, or adding or subtracting various Jewish rituals.

“There is a much greater openness to consider all aspects of ritual practice,” says Rabbi Eric Yoffie, president of the Union for Reform Judaism.

Is this Jewish fluidity good or bad for the Jewish community?

Bad, short-term; good, long-term, most experts agree. “A double-edged sword,” the AJCommittee’s Steven Bayme says. “It’s probably not good for [contemporary] institutional life,” says Gilbert Kahn, professor of political science at Kean University in Union, N.J “At the end of the day, it will be good for the Jewish community, because it will bring more people in, it will keep more people affiliated.”

He likens this to the havurah movement 40 years ago, which formed worship and study groups outside the denominational framework, bringing new blood into the Jewish community.

“To the extent that we are talking about people who are moving away from identification, observance, and affiliation of any kind, it is clearly bad,” says Rabbi Yoffie. “But to the extent that Jews are saying that they will no longer let their upbringing define their Jewish outlook, and want to consider all Jewish options, this is good for the Jewish community — even if it may be troubling in the short term for established institutions.”

Douglas Rushkoff, professor of media studies at New York University and an observer of American Jewry who wrote “Nothing Sacred: The Truth about Judaism” (Crown, 2003), says “I see the trend as good, since I am one of those who believes that Jewish continuity is defined by this sort of engagement.

“I feel that practicing by rote leads to the calcification of the tradition — that emptiness of unfelt and poorly understood ritual that sends people beyond Judaism for a sense of connection and meaning,” Rushkoff says.

Recognizing the increasing fluidity, several Jewish organizations have developed new programs to reach out to young, spiritually searching Jews.

The fall 2010 issue of Reform Judaism magazine describes several such programs conducted by Reform congregations around the country, including the “Sinai in the City” project of Temple Sinai of Roslyn Heights, Temple Shaaray Tefila’s JeTSeT (Jewish Twenties and Thirties at Shaaray Tefila) initiative in Manhattan, and the Next Dor DC activities coordinated by Rabbi Esther Lederman at Temple Micah in Washington.

Next Dor (dor is Hebrew for generation), under the auspices of the non-denominational Synagogue 3000 organization, is one year old, featuring one-on-one encounters like Shabbat dinners and holiday celebrations in such settings as cafes and restaurants and participants’ homes. The goal, according to the Synagogue 3000 website, is, “a network of synagogue engagement [that] connects young people not just to stand-alone social or cultural events …. but to the institution of the synagogue.”

“The goal is not to build membership, but to build Jewish community,” Rabbi Lederman, who formerly served at Congregation B’nai Jeshurun in Manhattan, told Reform Judaism magazine. “We’re meeting people where they are.”

“They’re not going to come to us,” says Rabbi Jonathan Blake of the Westchester Reform Temple, a Scarsdale congregation that this summer began running Next Dor activities, with the help of a grant from UJA-Federation of New York, for young Jews, former members of the temple or children of current members, in Manhattan.

“We’re changing the paradigm,” Rabbi Blake says. “We need to create a synagogue without walls for synagogue engagement. There are people who want to engage in Jewish life. They just haven’t found out how.”

When these 20- and 30-somethings start raising families, they plan to give their children similar religious choices, they say. “Expose, not impose,” says Ruvym Gilman, the son of émigrés from the former Soviet Union who works as legal and business affairs manager for Birthright Israel NEXT.

His parents received no Jewish education in their homeland, so could not pass on Jewish traditions to him. A Birthright alum, Gilman has studied about Judaism; he will be able to give his children Jewish choices one day, he says.

Abby Sher, the mother of two infants, has the same philosophy. She has started to bring more Jewish practice, like Shabbat dinners, into her home.

The author of last year’s “Amen, Amen, Amen: Memoir of a Girl Who Couldn’t Stop Praying (Among Other Things)” (Scribner), Sher says her spiritual life is still a curve. As her children grow older she says, she will teach them about the Jewish choices she has made. “I hope I’m not dogmatic at all,” Sher says. “I hope they explore every avenue.”

E-mail: steve@jewishweek.org