You can be an atheist and remain open to changing your mind if you see convincing evidence. God
could come down from the sky and telepathically communicate his message to every human being on earth at the same time.. There could be a mass rapture-like event where non-believers are punished and believers are taken to heaven. if something like that happens I wlll have to stop being an atheist because of new evidence.
Atheism is not ruling out every possibility of a god existing; you are an atheist when you are
unconvinced by traditional arguments for God's existence. You say "I don't believe God exists," not "I believe no God exists."
See:
Russel's Teapot.