The billboard -- one of several posted in the Sacramento area -- originally read: "Are you good without God? Millions are." Someone spray-painted the words "also lost?" beneath "millions are," CBS13 reported Tuesday.
Rachael Harrington of the Sacramento Area Coalition of Reason -- which paid to have the billboards put up -- said the ads are intended to let atheists and agnostics know they are not alone, the TV station said.
"This shows loud and clear just how necessary our message is, because prejudice against people who don't believe in a god remains very real in America," Harrington said.
The organization has asked Clear Channel, the company that owns the billboard, to file a police report. The company offered to replace the sign free of charge, Harrington told the TV station.