As a staunch atheist, how do I respond to people that believe that "God" is the only thing in their lives?

Personally I thing that "God" is a m delusion and religion is a control mechanism that has been used since Roman times but how do I relate to the bible thumpers that believe that everything that happens, happens for "God." Personally, I think it is absolutely ridiculous and that they are wasting their lives in devotion to at best a ghost. Normally when someone talks about God I just kind of nod and don't say anything. I don't want to get into a big debate but how can I acknowledge their feelings while expressing my own?