Arthur Mijares never saw it coming when he filed the federal paperwork to change the name of Contra Costa County’s most famous landmark from Mt. Diablo to Mt. Reagan.

It’s not that he’s such a big fan of the 40th president of the United States. It’s just that he believes, as a devout Christian, that naming a peak of such beauty and importance after the devil -- even in Spanish -- is “derogatory, pejorative, offensive, obscene, blasphemous and profane.”

“I just happen to be an ordinary man that worships God,” Mijares said by way of explanation. “He gave me this task in my prayer time. I said, ‘Lord, they’re going to think I’m a loon.’ ”

Mijares didn’t know the half of it.

In less than a month, more than 80,000 people have joined a Facebook group called “People AGAINST Re-naming Mt. Diablo to Mt. Reagan!!” The Contra Costa County Board of Supervisors, which will vote on the name change Tuesday, has been flooded with e-mail; the heated response runs nine to one against the idea, said Supervisor Susan Bonilla, whose district includes the beloved mountain.

Online comments -- and there have been thousands -- range from the sacred to the quite profane. It’s hard to figure out just who Mt. Diablo’s legions of supporters think is the real devil here: the gray-haired, retired rehabilitation counselor who would mess with history or the president he hopes to honor.

“I absolutely agree that Reagan was the Devil, and any monument that is dedicated to him is an absolute disgrace to anybody’s sense of moral fortitude,” fumed one post. “Per Nancy, just say ‘no.’ ”

Spluttered another, “What bozo came up with this idea?”

Even the rare Gipper lovers to chime in were dubious. “I’m a huge fan of Ronald Reagan,” posted one, “but this is just nuts.”

Christopher Kerr, a 25-year-old musician who plays drums, percussion and guitar in a band called the Orangutangos, started the Facebook group the last Thursday in January, because he was so upset that someone would even consider tampering with the 3,849-foot peak, from which 35 of California’s 58 counties are visible.

Then he headed to Fresno to visit friends for the weekend, Satan vs. Reagan the last thing on his mind. He didn’t check Facebook again until returning Monday, he said, “and there were already a couple thousand people. Whoa, what happened?”

What happened was that Kerr -- who describes his own political views as “I’m not a left wing or a right wing, I’m the whole . . . eagle” -- had touched a chord with two discrete groups of people: The ones who love Mt. Diablo and its history and the ones who hate the Great Communicator.

Here in the San Francisco Bay Area, both are far from endangered species. And they’ve been banding together on the Facebook page at an average rate of 3,200 per day.

Mijares, who can see Mt. Diablo from his home in the East Bay suburb of Oakley, said he’s not surprised to be so forcefully outnumbered. And he’s unfazed by the online vitriol directed his way.

“The people who don’t like the change tend to be a little bit more vocal,” he said. “You know what the difference is? Good and evil. I’m not saying these people are evil. There’s enough ungodliness there that they say things that are not so pleasant and nice and kind and thoughtful.”

The twin-peaked Mt. Diablo, which dominates the East Bay landscape, is a sacred site to the Golden State’s Native American tribes. The Miwok believed the mountain was originally an island, “from which Coyote and his assistant, Golden Eagle, made the world as we know it,” according to American Indian Quarterly’s fall 1989 edition.

Its name has long swirled with controversy. As legend has it, in 1805, Spanish soldiers were chasing a band of Bay Miwok who had escaped from a mission and apprehended them in a thicket at the base of a dramatic mountain. Darkness fell, and the Miwok disappeared.

When day broke, the mountain was shrouded in fog, and the soldiers realized that they’d been duped. So they dubbed the area Monte del Diablo, Thicket of the Devil.

“The name was transferred to the peak by non-Spanish explorers who associated ‘monte’ with a mountain and applied the Italian form Diavolo or Diabolo,” according to the Save Mount Diablo website, which is dedicated to preserving open space on and near the mountain. Monte del Diablo first appeared on an 1824 map.

One hundred eighty-one years later, Mijares applied to the U.S. Board on Geographic Names to change Mt. Diablo to Mt. Yahweh, among other possibilities, because Diablo was “profane” and “derogatory.” The board didn’t buy it. So he filed again in 2009, this time requesting that the peak commemorate the former president. Another applicant has filed to change the name to Mt. John Muir.

Contra Costa County supervisors are to vote on both requests Tuesday, and the federal names board will then take up the issue.

“The name is historical,” said Seth Adams, director of land programs for Save Mount Diablo. “Even though it’s a linguistic mistake, it’s got 200 years of history behind it. . . . Mt. Diablo is a special place in which people have invested huge reverence and love. They love it the way it is.”

maria.laganga@latimes.com