I think Atheism has nothing to do with the Dajjal --- idol worship and shurk are (MUCH) bigger sins than Atheism or being Agnostic, which can be remedied differently and properly with each individual and his/her reasons behind doubting the existence of a creator. The Dajjal would be having people that believe without thinking (unthinking sheep followers) and therefore are easily led astray to the false belief that worshiping him is what God wants them to do.