By fighting Hillary Clinton to a virtual draw this week in Iowa, Bernie Sanders became the most successful Jewish presidential candidate in American history. But that’s largely beside the point. Sanders’ real cultural significance isn’t that he’s Jewish; it’s that he’s secular.

Opinions about Jews no longer constitute a dividing line in American politics. Jews are well represented within the upper echelons of both parties. 95 percent of Republicans and 92 percent of Democrats say they would vote for a Jew for President. When Al Gore made Joe Lieberman, an Orthodox Jew, his running mate in 2000, it boosted him in the polls.

But that’s partly because while Lieberman’s brand of religion distinguished him from most Americans, his devotion to it did not. Except, tragically, when it comes to Muslims, Americans tend to respect any species of religious faith. The United States today is divided less between religious denominations than between those who practice any form of religion and those who do not.

That’s because although the United States remains more religious than other advanced democracies, it contains a growing secular wing. The share of Americans who refuse any religious affiliation has risen from one in 20 in 1972 to one in five today. And secular Americans increasingly identify themselves as Democrats. In 1995, according to the Pew Research Center, religiously unaffiliated Americans were nine points more likely to be Democrats than Republicans. By 2014, the gap was 26 points. Twenty eight percent of Democrats now eschew any religious affiliation, which means they outnumber any single religious denomination in the party. In the GOP, by contrast, evangelical Christians outnumber secularists by almost three to one.

Bernie Sanders is now the symbol of this divide. Republican presidential candidates talk about their religious faith incessantly. After finishing third in the Iowa Caucuses on Monday night, Marco Rubio thanked “my lord and savior Jesus Christ.” When Ted Cruz took the stage, the first words out of his mouth were “to God be the glory.” Even Donald Trump, who can’t accurately name the books of the Christian Bible, pretends to be devout.

Sanders, by contrast, said last fall that, “I am not actively involved with organized religion.” Culturally, this is part of what links him to his supporters. Iowa and New Hampshire may not have many Jews, but they lots have of secularists. It’s also part of what links Sanders to the young, who favored him over Hillary Clinton in Iowa by an astonishing 70 percentage points. Sanders may be in his seventies, but his rejection of organized religion is far more typical of Millennials than of people his own age.

Sanders also typifies what is happening in a large swath of Jewish America. Twenty percent of self-described American Jews now say they have no religion. Among American Jews born after 1980 that rises to almost one in three.

Politically, these secular Jews have much less in common with Orthodox Jews than with other secularists. Sanders has now replaced Jon Stewart as their most prominent symbol.

It’s unlikely Sanders will win the nomination. But sooner or later, if trends inside the Democratic Party continue, a secular candidate will. Whether or not that candidate is Jewish, his or her secularism will constitute America’s cultural frontier. Today, only 58 percent of Americans say they would vote for an atheist for president. (A lower percentage than say they would vote for a Muslim). Given the greater secularism of America’s young, that number should rise but it will remain a massive hurdle for years, if not decades to come.

Bernie Sanders’ success in Iowa thus poses a fascinating cultural question. The question is not whether Americans would elect a Jew. They would. It’s whether Americans would elect a Jew who eschews Judaism. The answer divides the two parties today, and may well for a very long time.