All mobster say to be catholic but the money is their god and the 7 sins are his apostols. So for answer to your question: are all atheists because don't believe in no god apart the money god.
John Gotti admitted to being an atheist while in prison. Mad Sam DeStefano was also an admitted atheist. I'm sure there are others, but many compartmentalized their Catholic faith and justified what they did. Joe Profaci and Joe Bonanno, for example, were active Catholics. Lucky Luciano wasn't religious, but I'm not aware of him discussing his personal beliefs.
u can add Nicodemo Scarfo snr there is CREEP here on this board who wrote him a letter and Nicky WROTE BACK thanking for the nice letter addin he is not into any religeous THING. Vic Amuso too the reason they get along so good