One of the nation’s largest atheist groups on Monday called for the resignation of Democratic National Committee CFO Brad Marshall, following revelations that he’d wanted to tarnish Sen. Bernie Sanders (I-Vt.) by questioning his faith.

“Entertaining such a cynical and bigoted line of attack violates any number of basic American principles: It presumes a religious test for holding office, something expressly prohibited in the Constitution,” said leaders of the Center for Inquiry, a reason-based organization that champions secular society, in a release.

In a May email leaked last week, Marshall appeared to question Sanders’ Jewish identification and suggest he might instead be an atheist. While he didn’t call out Sanders by name, Marshall referred to upcoming presidential primary contests at the time.

“Does he believe in a God,” wrote Marshall. “He had skated on saying he has a Jewish heritage. I think I read he is an atheist. This could make several points difference with my peeps. My Southern Baptist peeps would draw a big difference between a Jew and an atheist.”

The Center for Inquiry says this language is a divisive jab against a marginalized minority group, and exacerbates unfair stereotypes about people who don’t believe in God.

“The Democratic National Committee must make immediately clear that it finds Marshall’s line of thinking unacceptable, and that it will not countenance party operatives proffering attack strategies based on this kind of anti-atheist bigotry,” said Center for Inquiry CEO Robyn Blumner and other top executives.

Sanders had repeatedly and consistently answered questions about his faith at the time of Marshall’s email, suggesting the DNC’s CFO may not have been paying attention.

“I think everyone believes in God in their own ways,” Sanders told the Washington Post in a January interview. “To me, it means that all of us are connected, all of life is connected, and that we are all tied together.”

While many atheists had pointed out that this description seemed compatible with systems of non-religious thought, like humanism, Sanders never denied the existence of a supernatural God.

The Center for Inquiry says comments like Marshall’s are offensive to the rising number of religiously unaffiliated people in the U.S. This group, often referred to as the religious “nones,” includes atheists, agnostics and others who do not identify with any formal religion. They now make up nearly a quarter of the population.

“Let us be clear: Atheists are Americans,” said the Center for Inquiry executives. “Atheists are Democrats, as well as Republicans and independents. We are as integral a part of the fabric of this country as any other group. Atheists have fought, struggled, and died for this country and its values alongside friends, family, and neighbors of all beliefs. At every turning point for social progress, atheists, humanists, and other nonbelievers have been on the front lines, helping to lead the charge for civil rights, women’s equality, and LGBTQ rights.”

Marshall has apologized for his remarks, calling them “insensitive” and claiming that they “do not reflect” his beliefs or those of the DNC. But the Center for Inquiry says the response from Democratic leaders, as well as their candidate, former Secretary of State Hillary Clinton, needs to be stronger.

“Atheists, humanists, ‘nones,’ and all others who have rejected traditional religious belief will no longer be silent when institutions of power attempt to reinforce pernicious stereotypes about the nonreligious,” said Blumner and her colleagues. “We matter, we have a powerful voice — and a vote — and we will use it.