FORT WORTH, Texas — Stand on a corner in this city and you might get a case of theological whiplash. A public bus rolls by with an atheist message on its side: “Millions of people are good without God.” Seconds later, a van follows bearing a riposte: “I still love you. — God,” with another line that says, “2.1 billion Christians are good with God.”

A clash of beliefs has rattled this city ever since a group of atheists bought ad space on four city buses to reach out to nonbelievers who might isolated during the holiday season. After all, Fort Worth is a place where residents commonly ask people they have just met where they worship and many encounters end with, “Have a blessed day!”

“We want to tell people they are not alone,” said Terry McDonald, the chairman of Metroplex Atheists, part of the Dallas-Fort Worth Coalition of Reason, which paid for the atheist ads. “People don’t realize there are other atheists. All you hear around here is, ‘Where do you go to church?’ ”

But the reaction from believers has been harsher than anyone in the nonbelievers’ club expected. Some black ministers organized a boycott of the buses, with limited success. Other clergy members are pressing the Fort Worth Transportation Authority for a ban on all religious advertising on public buses. In addition, a group of local businessmen paid for the van with the Christian message to chase the buses with atheist messages around town.

“We just wanted to reach out to them and let them know about God’s love,” said Heath Hill, president of the media company that owns the van and one of the businessmen who arranged for the Christian ads. “We have gotten some pretty nasty e-mails and phone calls from atheists. But it’s really just about the love of God.”

The faceoff here follows efforts in other cities by several coalitions of atheists — American Atheists, the United Coalition of Reason and the Freedom From Religion Foundation, to name a few — that have mounted ad campaigns to encourage nonbelievers to seek out others of like mind. Some have compared their efforts to the struggle of homosexuals to “come out” and win acceptance from society.

In New York City, a large billboard promoting atheism at the entrance of the Lincoln Tunnel, which a local affiliate of American Atheists paid for, has generated controversy. (The message: “You know it’s a myth. This season, celebrate reason!”)

The Fort Worth group is affiliated with the United Coalition of Reason, whose local chapters have bought bus ads in Detroit, northwest Arkansas, Philadelphia and Washington D.C., as well as billboards in more than a dozen cities, among them Chicago, Houston, New Orleans, Seattle and St. Louis. Most of them are a blue sky with variations on the message: “Don’t believe in God? You are not alone.”

The ads have incited anger in some places. Vandals destroyed two bus ads in Detroit, ruined a billboard in Tampa, Fla., and defaced 10 billboards in Sacramento. One billboard in Cincinnati was taken down after the landlord received threats.

The local rapid transit authority in Des Moines, Iowa, pulled atheist ads off its buses in August 2009 because of complaints from local religious leaders. Four days later, however, the authority reversed its position after the local group that had bought the ads threatened legal action on First Amendment grounds.

In late 2008, a similar campaign of atheist ads was mounted on about 800 buses in Britain.

But nowhere has the reaction of believers been as forceful as in Fort Worth, to the delight of Fred Edwords, the national director of the United Coalition of Reason.

The coalition’s local chapter spent only $2,400 for four bus ads, which will run through the month in a city with about 200 buses.

“That’s more brouhaha for the buck than we have seen anywhere,” he said.

Some of the fiercest criticism has come from black religious leaders. The Rev. Kyev Tatum, president of the local Southern Christian Leadership Conference, has called for a boycott of the buses, saying the ads are a direct attack during a sacred time on the Christian calendar.

“It’s a season to share good will toward all men,” he said. “To have this at this time come out with a blatant disrespect of our faith, we think is unconscionable.”

While Tatum and about 20 other local pastors have urged their congregations to avoid the buses, a smaller group met recently with the transportation authority’s president to demand the policy allowing religious advertising on buses be reversed last week at a meeting of the authority’s board. Dallas’ bus system, for instance, enforces a ban on all religious ads and rejected the atheist messages.

“I’m not against them getting their message out,” said the Rev. Julius Jackson, the pastor at Macedonia Missionary Baptist Church. “I just don’t think it should be on public transportation.”

Dick Ruddell, the president of the Fort Worth Transportation Authority, said churches were free to advertise as well. The only ads not accepted, he said, are those that have to do with a few vices, such as cigarettes and alcohol.

“There is nothing in the policy about religious content,” he said.

Not all religious leaders are offended, and some are taking the high road.

“It doesn’t seem to me as an in-your-face, God-is-not-good message,” said Tim Bruster, the senior pastor at the First United Methodist Church, where 3,500 families worship. “My very strong opinion is that, as people of faith, the very thing we should not do is lash out and condemn.”

McDonald, chairman of the local atheist group, said the ad was never intended to insult Christians but to console atheists. He said the initial plan was to run the ad on the Fourth of July, which is why the ad features dozens of portraits of Texas atheists in an American flag motif.

Raising money and pulling together photos took longer than expected, he said, and the ad was not ready until last month.

“It can be pretty lonely for a nonbeliever at Christmastime around here. There is so much religion,” he said. “We thought, ‘What the heck? Nobody owns December.’ ”