FORT WORTH  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 atheists bought ad space on four city buses to reach out to nonbelievers who might feel isolated during the Christmas 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 nonbeliever’s club expected. Some ministers organized a boycott of the buses, with limited success. Other clergy members are pressing the Fort Worth Transportation Authority to ban all religious advertising on public buses. And a group of local businessmen paid for the van with the Christian message to follow the atheist-messaged buses around town.