The message: "Don't believe in God? Join the club."
How Atheists see the message: "You're not the only one who doubts, disbelieves, and/or finds the whole thing silly. (If you do believe, this message isn't directed at you.)"
How Certain Strains Of Christian see the message: "SATAN IS COMING FOR YOUR CHILDREN!!! WE'VE BEEN INVADED BY UNBELIEVERS! CLEARLY THESE ARE THE LAST DAYS! PRAY NOW! PRAY HARDER! KEEP PRAYING!!!"
The human capacity for hysteria never fails to amaze me.
Also, to all the other believers out there, who saw or read about those signs, and thought "Hey, atheist billboards. Don't see that every day," or "Guess I won't join that club," or "Well, atheists enjoy spending time with like-minded people, too," or even "So, what?" ...Thank you for being reasonable.
h/t Personal Failure.