Unknown vandals unhappy about atheists' billboard in Charlotte, N.C., spray-painted "Under God" on the ad, the city's atheist association discovered Monday. The defaced message will remain in place until after July 4, the group reports, which is the soonest that workers can furnish a fresh billboard image. Here's how the vandalized billboard now looks:

The billboard reads, "One Nation Indivisible," which is the phrase preceding the 1954 insertion of the words "under God" to the Pledge of Allegiance, reports the Charlotte Observer's Tim Funk. The billboard was erected on Billy Graham Parkway last week. (Graham is, of course, the state's famous evangelical preacher.)

Similar North Carolina ads have gone up in Asheville, Greensboro, Wilmington, Raleigh, and Winston-Salem as a Fourth of July project by the area's atheist association. The group has filed a police report and will replace the billboard.

“It was done by one or two people off on their own who decided their only recourse was vandalism rather than having a conversation,” Charlotte Atheists & Agnostics spokesman William Warren said. “It does show how needed our message is. As atheists, we want to let people know we exist and that there’s a community here.” Warren told the Observer when the sign first went up that its location wasn't intended as a rebuke to the Rev. Graham.

He said the group has added more than 50 members since the ad went up. Atheist ads are often a target for vandals. Three of 10 atheist billboards erected in Sacramento, Calif., were defaced in February, and a series of atheist bus ads was recently vandalized in Detroit.

According to a 2007 study by the Pew Research Center, about 6 percent of Americans are secular. Less than 2 percent of all Americans identify as atheist.

An act of Congress changed the language of the Pledge of Allegiance during the height of the Cold War. Francis Bellamy, a Baptist minister and Christian socialist, composed the original pledge in 1892.

— Liz Goodwin is a national affairs reporter for Yahoo! News.