Instead, what about saying you’re a Catholic apostate? It sounds vaguely religious. It also sounds less confrontational, even though an apostate is someone who’s abandoned their religion. You’re offering the possibility of a shared history. Declaring that you’re an atheist to someone who isn’t is dropping a turd in the punchbowl of conversation.
There’s a cultural component to saying “I’m a Catholic apostate.” You’re letting the other person know you’re connected culturally to other Catholics through similar experiences. You might not be in the tribe anymore, but you still remember stuff like the secret Catholic handshake and harrowing encounters with nuns.
It could be my imagination, but lots of atheists love to argue. There’s not much to argue about. You’re either on the bus or off the bus. Hopefully by switching Catholic apostate for atheist you’ll signal you’re not spoiling for an argument, and might be looking for common ground.
Too bad that the next time you’re filling out a form and need to declare your religion there won’t be a box to check for Catholic (or whatever religion you’ve left) apostate.