The very first right laid out in the First Amendment of the Constitution is not about speech, the press, peaceable assembly or petitioning the government. It addresses the freedom of religion -- establishing a right that, among other protections, prohibits the government from favoring one faith over any other or giving preference to religion over nonreligion.

That frequently contested clause is being put to the test in Arizona this week, where Democratic state Rep. Juan Mendez, an atheist, says he has been stopped from leading the state House of Representatives in an opening prayer because he wants to do so without mentioning God.

Last month, state House Majority Leader Steve Montenegro, a Republican, issued a memo that his office said was meant to serve as "written guidance" on the prayer issue. In it, he defined prayer as "a solemn request for guidance and help from God, according to the Arizona Capitol Times .

While Montenegro's office maintains that the official policy hasn't changed, Mendez sees it as the latest move to exclude him from the ritual.

"They're not officially barring me from doing it," Mendez told The Huffington Post. "Every time I have my assistant ask to do the prayer, they just tell me that I can't schedule that day and that day is taken by somebody else."

Stephanie Grisham, a spokeswoman for Montenegro, said that "all of the slots for prayer were simply full," but wouldn't say whether Mendez would be allowed to lead the prayer if he signed up for an open slot.

In the past, each state representative was assigned a day to lead the prayer, regardless of religion. Mendez used his opportunities to deliver secular invocations that appealed to the chamber's shared humanity and lauded the work of famed astrophysicist and educator Carl Sagan.

While Mendez says he was initially hesitant to speak on those occasions because he knew his beliefs were in the minority, he says he now believes it's necessary to occasionally offer a perspective grounded in worldly reason, rather than supernatural faith.

"Before this all started, I would rather we not have done the prayer at all," said Mendez. "But now, I see that it at least has value for my constituents. So if we're going to do it, I would at least want it to be fair."

Mendez says he's now been blacklisted from the tradition, and wasn't given an opportunity to lead a prayer ceremony at all last year. Last week, he was forced to deliver the invocation he'd planned during the personal comment period after the official prayer. He discussed the need for prayer to reflect people of different religions as well as those without a religion. But the rest of the chamber was preoccupied with official business, Mendez says, a far cry from the quiet and captive audience he would have encountered during an official prayer.

The prayer process had become discriminatory and alienating to the steadily growing number of Americans who are nonbelievers or otherwise unaffiliated with a religion, Mendez said.

"I'm not trying to mock them. It's a custom that they do. They asked me to be a part of it in the beginning. I'm trying to be a part of it now," said Mendez. "But they're drawing these guidelines that I just can't fit into, so I'm relegated to having to participate in a totally different manner. It's definitely not equal."

While Mendez said he felt the policy was disrespectful, others say it's unconstitutional as well.

"What Montenegro is doing is imposing a rule that excludes not only atheists, but also some minority religions. You know, Buddhism doesn't necessarily address a higher power," said Andrew Seidel, an attorney for the Freedom From Religion Foundation, a nonprofit that fights for the separation of Church and State.

The 2014 Supreme Court decision in Town of Greece v. Galloway ruled that religious prayer in government meetings is constitutional as long as there is a policy of nondiscrimination, a ruling that the FFRF says includes nonbelievers. Critics of public prayer in the Phoenix City Council pointed to this ruling earlier this month and eventually pressured officials to scrap the tradition after a pair of Satanists signed up to deliver an invocation .

"[Greece v. Galloway] specifically addresses this exact question and says that Montenegro's rule is unconstitutional," Seidel said.

Montenegro's office said that the memo was written by their general counsel, and that both he and Montenegro were "comfortable with its contents."

Seidel also pointed out that in this case, nobody was telling lawmakers they couldn't pray. He suggested that lawmakers who want to preserve the strictly religious nature of the opening invocation were making a political calculation.

"The reason that they're organizing the prayer and promoting it using their government offices is because they want to be seen to be praying by their constituents," he said. "It's political pandering of the worst kind, and it not only denigrates the Constitution and the government, but it also denigrates the religion that they are promoting."

Seidel then offered up a passage from Matthew 6:5 that Montenegro, a Christian minister, might be familiar with.

"And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men."

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