(CNN) Nobody would mistake Bernie Sanders' stump speech for a sermon. When making moral arguments about income inequality or climate change, he's more likely to quote statistics than Scripture.

Though raised Jewish, Sanders says that he is "not particularly religious," nor is he a member of any congregation or synagogue. "I am not actively involved in organized religion," he has told reporters

But at a CNN town hall in New Hampshire in February, Sanders seemed to contradict himself.

"It's a guiding principle in my life, absolutely," said the Vermont senator and Democratic presidential candidate.

"You know, everyone practices religion in a different way. To me, I would not be here tonight, I would not be running for president of the United States if I did not have very strong religious and spiritual feelings."

So what gives? Is Bernie Sanders religious or not?

Many Americans answer that question with a shrug, according to a January poll conducted by the Pew Research Center . Nearly a third say Sanders is "somewhat" religious; nearly a third say he's not, and more than a quarter say they don't know.

It may seem impolite to question Sanders' religious views. Who cares whether he spends his Saturdays at a shul or a socialist rally? Millions, meanwhile, have rallied behind his presidential campaign, cheering his jeremiads against consumerism and political corruption.

But if Sanders wants to win the Democratic nomination, he has to expand his base, which means luring churchgoing Christians away from Hillary Clinton . According to exit polls, Clinton has dominated Sanders among Americans who attend worship services weekly, leading to a "God gap" among the Democratic candidates.

Sanders seems to know this. He has toured black churches and addressed evangelicals at Liberty University in Virginia. This Friday, just days before the New York primary, Sanders addressed a Vatican conference on the morality of market economies.

But speeches alone won't attract religious voters to Sanders, said Marvin A. McMickle, director of the Black Church Studies program at Colgate Rochester Crozer Divinity School in New York.

"We hear the words, but words can be memorized. I want to know more about the sources of those words. I want to know the rock he is standing on."

That rock is hard to find. Sanders rarely talks about religion and grows impatient with reporters who try to pry open his soul or delve into his Jewish background. He doesn't belong to a prayer group in Congress or a congregation in Burlington. By conventional standards, Sanders may be the least religious candidate to run for president in quite some time.

But Sanders' political platform is unmistakably moral, scholars say, and it draws more deeply on faith teachings than even he may know. In other words, if Bernie Sanders is standing on a rock, it may point to the Promised Land of socialist Scandinavia, but it sits on ancient soil.

Torah 101

Eli Sanders, Bernie's father, fled poverty and prejudice in Poland, immigrating to the United States in 1921. Crowded into a cramped Brooklyn apartment, his young family held Passover seders but rarely attended services at the local synagogue.

The Sanderses were proud of their Jewish heritage, the senator and his brother have said, but belief in God wasn't an abiding concern.

Back in Poland, Eli Sanders' family paid a high price for their Judaism. Many were killed in the Holocaust, sending a searing message to the next generation.

"A guy named Adolf Hitler won an election in 1932," Sanders said last year , responding to a question about how Judaism informs his political views. "He won an election, and 50 million people died as a result of that election in World War II , including 6 million Jews. So what I learned as a little kid is that politics is, in fact, very important."

(There is some dispute about Sanders' framing of Hitler's rise here. Hitler lost his election in 1932, but his Nazi Party won seats in the Reichstag, setting the stage for Hitler's dictatorship.)

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Sanders and his brother, Larry, attended Hebrew school at an Orthodox synagogue. They studied the Torah, reading about the patriarchs and prophets. It was more Torah 101 than Advanced Talmudic Studies, but the lessons stuck, Larry has said.

For the Sanderses, politics takes precedence over piety. In his autobiography, "Outsider in the House," Sanders recalls tagging along with Larry to meetings of Young Democrats at Brooklyn College. (Like Bernie, Larry Sanders made a career in politics; he is now the health issues spokesman for the Green Party in England.)

At the University of Chicago, where Bernie Sanders went to college, he spent hours imbibing socialism's sacred canon: the writings of Karl Marx, Leon Trotsky and Eugene V. Debs, a socialist who ran for president five times.

In his autobiography, Sanders completely skips the next chapter in his life. There is no mention of the months he spent on a kibbutz in northern Israel. (Actually, Israel isn't mentioned at all, a noteworthy omission for a Jewish-American politician.)

Friends say the agrarian Israeli collective solidified Sanders' faith in socialism, but it apparently did not deepen his relationship with organized Judaism

As Sanders moved to Vermont and began his political career, local rabbis learned not to invite him to services. He never accepted, though friends say he would attend Jewish friends' funerals at synagogues. He wasn't hostile to religion, colleagues say. More like indifferent.

"He is quite substantially not religious," Larry Sanders would later say.

(Still, Bernie Sanders knows enough about Judaism to play a convincing cameo as "Rabbi Manny Schewitz" in a 1999 romantic comedy.)

When Sanders came to Washington in 1990 as a congressman, he kept his distance from the city's politically active Jewish community.

Rabbi Jonah Dov Pesner, director of the Washington-based Religious Action Center of Reform Judaism, said he tried to engage Sanders in a conversation about Judaism on a bus ride to Selma, Alabama, for the 50th anniversary of Bloody Sunday . Sanders politely rebuffed the rabbi's inquiries.

"He just didn't want go there. He obviously is not a practicing Jew, and doesn't want to talk about it, and I respect that."

Instead, Sanders was known in the Capitol as a stern-faced figure stuck in the political wilderness. In speech after speech, he harangued high officials, castigated his congressional colleagues, and even alienated allies by fulminating against the influence of money in politics.

"He is out there wailing on his own," a fellow New England Democrat told The New York Times in 2007

But Sanders wasn't quite that isolated.

The media flocked to the democratic socialist -- a rare bird in American politics -- especially after he was elected to the Senate in 2007. Sanders tried to shoo reporters away, saying personality profiles don't interest him. He was especially reluctant to speak to Jewish publications about his religious background.

But after Sanders began his presidential campaign, he could no longer ignore those questions.

'I am who I am'

Sanders first hinted at his unorthodox theology in, of all places, a talk show interview. After questioning whether the socialist senator swims too far from the political mainstream to be elected president, ABC's Jimmy Kimmel asked Sanders if he believed in God.

"Well, you know, I am who I am," Sanders said, evoking God's evasive answer to Moses through the burning bush. "And what I believe in, what my spirituality is about, is that we're all in this together -- that I think it's not a good thing to believe, as human beings, that we can turn our backs on the suffering of other people."

"And this is not Judaism," Sanders continued. "This is what Pope Francis is talking about: That we cannot worship just billionaires and the making of more and more money. Life is more than that."

It was a short answer. Just 30 seconds of spirituality before Sanders retreated to his stump speech, bemoaning childhood poverty and a lack of family leave programs.

Susan Jacoby, a well-known secular historian, said Sanders avoided Kimmel's question.

"He weaseled out. It was the exact opposite of his usual blunt talk. It was the answer of someone who is a secular humanist but doesn't call himself that."

Secular humanism, briefly, bills itself as a "naturalist philosophy." It rejects the idea of deities and supernatural forces while embracing an ethics of cooperation and respect for personal freedom. "No god will save us," declares a humanist manifesto written in 1973. "We must save ourselves."

Jacoby, who has written about secularism in American history, says that Sanders may be the least religious presidential candidate since Abraham Lincoln. Though he was an ardent reader of the Bible, Lincoln refused to join a congregation. (Other historians argue that Lincoln may have been our most theologically astute president, despite his distaste for organized religion.)

The growing size of the "nones" -- Americans who are not religiously affiliated -- positions Sanders to become a bridge to the first openly atheistic presidential candidate, the kind of politician who has become common among European liberals, Jacoby said. The Vermont senator already borrows many of his political ideas from Europe, why not take their lead in publicly pronouncing his lack of religious faith?

The Sanders campaign has said he is not an atheist, pointing out that he has spoken of his belief in God on numerous occasions.

"I think when we talk about God, whether it is Christianity, or Judaism, or Islam , or Buddhism, what we are talking about is what all religions hold dear," Sanders said during a CNN town hall in March . "And, that is: To do unto others as you would like them to do unto you." In other words, Sanders says, his spirituality can be summed up by the Golden Rule.

But what Sanders doesn't say about God and faith can be revealing. He never speaks of an Almighty who blesses the United States or hears the prayers of Israel, nor a God who judges the quick and the dead.

"He explicitly does not say that he believes in an Abrahamic God who controls everything," said Paul Fidalgo, a spokesman for the humanist Center for Inquiry.

In fact, Fidalgo noted, if you listen closely, Sanders has said many times that he is proud to be Jewish, but has rarely, if ever, acknowledged Judaism as his religion.

Sanders' lack of religious orthodoxy sometimes peeks out in campaign events, such as a town hall hosted by Fox News in March, when he argued that health care is a human right.

"Where does that right come from?" interrupted Fox's Brett Baier

That kind of question turns most people angling for the White House into amateur theologians. Even the deist Thomas Jefferson argued that our most important rights come from a Creator.

But Sanders wouldn't take the bait.

"Being a human being" is the source of our rights, he said. Motioning to a woman in the audience, Sanders continued, "I believe that if she is poor and you are rich, she is entitled to the same quality health care you have, because she is a human being."

It was an answer that could be sung from the rafters in a humanist hymnal, and American "nones," especially millennials, are flocking to Sanders' moral-but-not-religious message. According to exit polls, Sanders' support has come largely from voters under 45 and people who rarely, if ever, attend worship services.

"I feel weird using words like values and morals," said comedian Sarah Silverman while introducing Sanders at a rally in Los Angeles last year, "because those are words that have been co-opted by people who wear them like shrouds to justify terrible things like bigotry and greed. I'd like to take them back tonight to describe Bernie Sanders. His moral compass and his sense of values inspire me."

Like Sanders, Silverman was raised in a Jewish family but is not involved in organized religion

Some atheists want Sanders to be more outspoken about his religious beliefs -- or lack thereof. But many also acknowledge the political liability of atheism. Most Americans (51%) say they would be less likely to vote for a candidate who does not believe in God.

But Gallup polls suggest Americans are even less likely to vote for a socialist candidate, and Sanders has already crossed that bridge.

"So if there's anyone who could defend the concept of atheism," said Kyle Kulinski , a host on the Secular Talk Radio Network, "it would be Bernie Sanders."

Unlike atheists, American Jews have been slow to embrace Sanders' campaign, even after he became the first Jew to win a presidential primary last February in New Hampshire.

Some say that Sanders isn't "Jewish" enough. Others argue that Joe Lieberman , who was more religiously observant, already broke the political barrier when Al Gore nominated him for vice president in 2004. Others say the Sanders candidacy is too quixotic to kvetch or kvell about.

In some ways, though, Sanders typifies American Judaism.

Many Americans Jews are, like the Vermont senator, proud to be Jewish, though not connected to a synagogue or temple, according to a Pew Research Center study conducted in 2013. Many also marry outside of the faith. (Sanders' wife, Jane, is Catholic).

Less than one in five American Jews say observing religious law is essential to being Jewish, and an even smaller percentage say Judaism is mainly a religious matter. The vast majority, like Sanders, say that remembering the Holocaust, leading an ethical life and working for justice are the touchstones of modern Judaism.

Propheteering

Bernie Sanders has been in Congress since 1990. But rather than catalog accomplishments from his time on the Hill, his campaign website lists a series of prophecies.

He voted against both wars in Iraq, arguing that they would sow chaos in the Middle East. He predicted that the North American Free Trade Agreement would be good for corporations but bad for blue-collar workers. He warned against lifting regulations on Wall Street 10 years before the crash of 2009.

The popular notion of a prophet is someone who can predict the future. A mix, maybe, between a fortuneteller, a weatherman and a pundit.

But the Hebrew Bible depicts prophets primarily as political gadflies, said Walter Brueggemann, a Christian theologian who has written extensively on the prophets. All of the early Jewish prophets, starting with Moses, were statesmen. Many were unsuccessful reformers, mosquitoes on the arms of complacent kingdoms.

Moses famously demanded that Pharaoh free his people, then delivered a divinely ordained set of new laws. Isaiah, Jeremiah and Amos chastised the Hebrew ruling class for wasting their money on pointless luxuries like fancy cedar offices while turning their backs on the poor.

"Economic injustice is a major theme -- if not the major theme -- of the prophetic tradition," said Brueggemann.

The prophets tended to emerge from the peripheries at moments of chaos and change, said the theologian, whose book "The Prophetic Imagination" has been a seminary staple since 1978.

In the name of God, prophets demanded righteousness, explicitly tying personal morality to public welfare. Rituals mean nothing when children starve outside the temple walls, they argued. But the prophets weren't after empty repentance. They wanted a revolution. They were the original protest candidates.

Brueggemann said he sees some of the prophets' fire smoldering in Sanders.

"He's cantankerous but his cantankerousness is checked by hope. He stands outside the ideology of capitalism and therefore can see more clearly what the system is doing to individuals. He is bearing witness to that, and offering an alternative vision."

Questions about whether Sanders' vision of a socialist America is unrealistic are beside the point, Brueggemann said. Prophets are not pragmatists, and the fog of futility often hangs over their complaints.

A prophet's real goal is to stretch our moral imagination, to bring the private pain felt by history's outcasts to public expression. That's something that Sanders has done in Washington for nearly three decades, even as few people listened.

"If you check the things that I've been saying, they're pretty much the same thing, for better or for worse, for the last 25 or 30 years," Sanders told Salt & Light, a Canadian Catholic media ministry.

Brueggemann said the moral foundation of Sanders' political crusade is profoundly Jewish, even if the candidate does not acknowledge it. "I suspect he's not schooled in the niceties of Jewish theology, but he has inhaled the tradition for so long that it may be intuitive for him."

Of course, there are major differences between Sanders and the prophets. A large part of the prophets' ministries, after all, concerned religion. The correct way to worship God, a rejection of false idols, how to honor and keep the Sabbath.

Sanders shows zero interest in those topics.

Jonathan Sarna, a professor at Brandeis University and the dean of Jewish-American historians, cautioned against drawing too strong a link between Judaism and Sanders' political platform.

"If that were the case, then the Orthodox (Jews) would be the most pro-Sanders. That's nonsense. I have grave doubts that if you gave Sanders a test on the prophets that he would pass."

Still, Sarna said, Sanders probably imbibed the prophets' lessons indirectly, through an unorthodox source: socialism.

Like Sanders' father, many Jews who emigrated from Europe in the 20th century were steeped in both Judaism and socialism. Many left Jewish rituals behind, in large part because of anti-Semitism and corrupt rabbis in their homelands, while clinging to Judaism's ethical teachings.

"To their minds, socialism became a kind of substitute religion," Sarna said.

In socialism, Jewish immigrants found a "secularized Judaism" whose central tenets were humanitarianism, solidarity and progress, a mix that might sound familiar to Sanders supporters.

The Book of Bernie

A few days before Sanders left for his speech at the Vatican, I talked to him about religion for a few minutes.

I asked what he had learned from Judaism's ethical teachings, pointing out that Pope Francis sometimes cites the Hebrew prophets in making a case for spreading the world's wealth.

"Well, I think it's all religions," Sanders said, beginning a refrain he's often repeated when asked about faith. "It's not just Judaism, it's not just Christianity, it's not just Islam or Buddhism. What all religions are saying is that one of the goals of life is to do unto others as you would like others to do unto you."

Ah, the Golden Rule again.

I tried to press further, asking the senator if he's ever thought about the resonances between the Hebrew prophets and his message on economic inequality.

"No," Sanders said quickly, before diving into a topic he actually enjoys talking about: Pope Francis.

"I am going to the Vatican because I am deeply impressed by the teachings of this Pope," he said. "He has shined a light for all the world to see, that we cannot continue to tolerate the kind of greed and selfishness that we are seeing in the global economy."

This was far from the first time Sanders praised Francis. I half expect him to have a Pope poster hanging in his Senate office.

His favorite papal quote seems to be from Francis' apostolic exhortation, "The Joy of the Gospel," in which the pontiff decries the "idolatry of money." In this country, we don't just worship the Almighty Dollar, Sanders says, we revere the millionaires and billionaires who accumulate wealth.

Sanders made that argument in his most religious speech, to young evangelicals at Liberty University, last fall. It was Rosh Hashanah, an important holiday on the Jewish calendar, though Sanders made no mention of it.

The speech was a hodgepodge of quotes from the Bible, Pope Francis and the ubiquitous Golden Rule. Magpie moralism, you might call it, taking a bit from each tradition to support an idiosyncratic faith.

Sanders also cited the prophet Amos at Liberty University, quoting his prayer that justice and righteousness would roll like a never-ending stream. It would be hard to argue that justice rolls in America, he told the students, launching into a litany of statistics demonstrating the gap between rich and poor.

But the subtext of Sanders' speech at the university came from a different Hebrew prophet, Isaiah. Sanctimony won't save you, Isaiah tells the Hebrews . You can't fast or pray your way into God's good graces, and religion means nothing if you don't share your bread with the hungry, bring the homeless into your house and clothe the naked.

Perhaps that's why Sanders is so impatient with questions about religion. To him, personal faith in God is almost irrelevant; everything rolls into the Golden Rule, and all the rest is commentary

And if you don't believe him, just ask the Pope.