The biggest laugh in my play “Rabbi Sam” comes early in the first act.

The congregation’s new rabbi is preaching his first sermon, chiding American Jews for being stuck in the past, for practicing “museum Judaism.” Then he goes a step further: “And you know what the worst is? When they throw in just a pinch of that shtetl kitsch! Tevye is not going to save us!”

That sometimes gets a laugh. But nothing like what comes next.

The lights shift, and we hear from Harriet, one of the synagogue’s board members. Speaking in a tone of disbelief, she says: “It took a minute to sink in. Does our new rabbi not like ‘Fiddler on the Roof’­?! ”

And the audience goes wild.

Comedy is a curious business. My collaborator, David Ford, and I wrote Harriet’s line almost as an afterthought. We were looking for counterpoint, not comedy. We wanted to keep bouncing back and forth between the rabbi and the board, to build dramatic tension.

But I’ll take the laugh! Rabbi Sam is a solo show. I play all the characters — Harriet and the rabbi and 10 others — and by that point I’m already a little winded. When the audience laughs, I get to catch my breath.

And I get to think. In those few seconds of laughter, I marvel at the universality of “Fiddler on the Roof.” My audience may know a lot of Torah or none, may be scholars of Jewish history or ignorant of it, but everyone knows “Fiddler.” Even the non-Jews know “Fiddler.”

And — that’s as far as the train of thought travels; the laugh dies down, and the show must go on.

But offstage, there’s more to think about.

Harriet is right: Rabbi Sam does not care for “Fiddler.” Nor does he love klezmer music or anything else that smells even faintly of romanticizing the Jewish past.

Early in my work on the play, I happened upon an audio cassette of a talk by an American rabbi. He was discussing the teachings of the Sfas Emes, the great 19th century Chassidic rabbi from Poland.

I felt myself being drawn in, not just to the wisdom of the teaching, but drawn into that lost world, picturing myself in the crowd of disciples huddled around the Sfas Emes — people like my own Eastern European ancestors, poor simple folk, dressed plainly, perhaps a little hungry, hanging onto the sage’s every word.

And then I awoke from the daydream and thought the dangerous thoughts of Rabbi Sam. The shtetl of your imagination is a black hole! Get sucked in and you will never come back out! Beautiful as the teachings may be, they were for poor people, pre-industrial, segregated from the rest of society, politically powerless! You are living in the wealthiest, most powerful country in the history of the world, you have a hot-water heater, and your vote affects the fate of the planet!

Until American Judaism squarely faces this existential truth, and builds a spiritual narrative true to our time and place, most American Jews will shrug their shoulders and synagogue attendance will dwindle.

It got worse. Rabbi Sam asked me if I thought Tevye and the Sfas Emes were more authentically Jewish than I.

He wanted to know why modern American Jews keep looking elsewhere for authenticity — to Europe, to the past, to Israel. Why can’t we be ourselves?

Writing “Rabbi Sam” was a three-year thought experiment into what a 21st century American Jewish narrative might sound like. Over and over the writing surprised me.

There was Torah and midrash in there, but also Thomas Jefferson and Abraham Lincoln and Susan B. Anthony. “America’s greatness,” says Rabbi Sam, consists of “acts of stepping out of the confines of what had seemed immutable. Which makes America what? A Jewish nation!”

Rabbi Sam wants American Judaism to be like jazz, an improvisation on tradition, and sermons to be riffs discovered in the moment, not essays read from a page. He wants a Judaism rooted in tradition but robust with new knowledge, where the Big Bang can wrestle with Genesis in the sanctuary.

And he thinks Judaism is so good, we should share it with the non-Jews. “When we fail to bring our non-Jewish friends into our Jewish experience,” he says, “we are creating a division in our heart — American on one side, Jewish on the other.”

I suppose I write plays because my characters can take me places I can’t otherwise go, can think thoughts and say things I am unable to.

Even now, with the writing long finished, Rabbi Sam’s words keep working on me, provoking me, amusing me, disturbing me. I agree with him, and disagree. But I know there is no going back.

Like Harriet, I love “Fiddler on the Roof”; like Rabbi Sam, I’m not counting on Tevye to save us.

Charlie Varon is a Bay Area playwright, comedian and actor. He returns to the Marsh theater in San Francisco for 11 performances of “Rabbi Sam” from Oct. 17 to Nov. 22. Information: www.themarsh.org.