The scientific method says that you can never prove anything to be a 100% true. Things are either likely or unlikely to be true. In my opinion a real atheist is a person who thinks the existence of any higher power is unlikely. Tomorrow if I started hearing voices or hallucinating, I would probably think it’s more likely that I was having a psychotic break rather than seeing god. Agnostics think that the existence of gods is probable. Your opinion on the matter has to be dubious for you to be an atheist.
Do gods exist?
Agnostic: I don’t know?
Atheist: I don’t know, but I think it’s unlikely.
If the latter is how you answer that question, then you need to come out. Pretending to be a dummy isn’t doing anyone any good.