Translated, it’s a “good news-bad news” sort of message:
“The bad news is, God doesn’t exist. The good news is, you don’t need him to.”
Right now, it’s only on two busses in Genova. But it will expand from there. I’d predict Milano before Roma but it will get there. Don’t look for this message on gondole in Venice, though — the decorations that can go on a gondola are tightly regulated. (More people take the vaporetti to get around there; the gondole are just for tourists.)
Auguri, humaniste italiane!