Thanks to GOP presidential candidate Sen. Ted Cruz of Texas, atheists are having their "Muslim moment."

In a discussion about the persecution of Christians in the United States and around the world at the National Religious Liberties Conference in Iowa on Friday, Senator Cruz declared, "Any president who doesn't begin every day on his knees isn't fit to be commander in chief of this nation."

The comment didn't go quite as far as Ben Carson's declaration that a Muslim shouldn't be president, but it serves to single out atheists and underscore the stigma against atheism that persists, even among candidates for the nation's highest office.

With the exception of socialists, Americans are least likely to vote for an atheist president, according to a June Gallup poll. A 2014 Pew Research Center study that asked respondents to rate how they feel about different religious groups on a scale of 1 to 100, with 50 representing a neutral opinion, found that atheists scored a chilly 41.

Another Pew study found that while 10 percent of Americans would be unhappy if a family member married someone of a different political persuasion, and about 30 percent would be unhappy with a gun-owner in-law, nearly 50 percent said they'd be unhappy if a family member married an atheist. In fact, one study found Americans regarded atheists as less trustworthy than rapists.

It's a bias that appears to be widespread and largely unchallenged. As Psychology Today points out, academic studies have demonstrated that atheist patients are given lower priority on organ donation lists, and atheist parents are more likely to be denied custody rights after a divorce. It is illegal for an atheist to hold public office in seven states, atheists cannot testify as a witness on trial in Arkansas, they aren't allowed in the Boy Scouts, and Humanist chaplains are barred from serving in the nation's military.

So severe is the stigma it's been dubbed "secularphobia," and many atheists describe identifying as nonbelievers as "coming out."

Why are atheists so unpopular?

In a culture in which religiosity remains for many synonymous with morality, atheism is regarded by many Americans as immoral. A 2014 Pew Research Center study found that more than half of Americans (53 percent) believe that belief in God is essential to morality.

That's backed up by a set of studies. In one, participants were told a story about a person who accidentally hits a parked car and fails to leave insurance information for the other driver. When asked how probable it was that the offender was a Christian, a Muslim, a rapist, or an atheist, participants thought it equally likely the culprit was an atheist or rapist, and unlikely he or she was a Christian or a Muslim.

In another study, participants were asked to choose between an atheist or a religious candidate for a job. For high-trust jobs like daycare workers, people were more likely to prefer the religious candidate over the atheist.

Many people distrust atheists because they believe people behave more morally when they think God is watching them.

There's another reason atheists may be unpopular: there is no stigma associated with atheist prejudice. "Where there is a stigma ... attached to being racists, or anti-Semitic, or Islamophobic, or homophobic, there has never existed a social or cultural backlash against people who openly express disdain for secular folks," writes Pitzer College sociology professor Phil Zuckerman for Psychology Today.

Of course, other minority groups have faced, and overcome, biases. What would it take for atheists to do so?

Charitable acts done in the name of atheism – like Richard Dawkins' Non-Believers Giving Aid campaign, launched after the Haiti earthquake – may help.

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And studies that have found that political insecurity leads to increased religiosity also suggest the opposite is true: high levels of social and political security, like those found in Scandinavian countries, may lead to lower levels of religiosity, and lower levels of bias against nonbelievers.

Finally, it may simply come down to demographics. Increasing numbers of young Americans identify as "religiously unaffiliated." Unlike Senator Cruz and his cohorts, they are far more likely than their parents or grandparents to accept atheists – as friends, spouses, even as presidents.