Typically, if this statement is used in an argumentative context, the "wannabe atheists" who are condescending in nature tend to have a list of scientific facts ready to use however they wield nothing more than an elementary school biology, physics and chemistry education.
REAL Atheists do not use this common line because they know they are not religious, they do not care for religion and they don't want to waste time arguing about religion. Point being, if you use this line and claim to be atheist, you aren't. You're no better than an extremist. Please re-evaluate your provocative nature and stop using "I'm an Atheist" you give a bad name to the REAL atheist community who truly does not care one bit about religion.
Example of Random Usage
Guy 1: Dude did you watch how I met your mother?
Guy 2: Yeah, but I'm an atheist so......
Example of Provocative Usage
Guy 1: My god I can't believe what happened yesterday in the Liverpool vs Chelsea game.
Guy 2: Yeah you see, I'm an Atheist, god doesn't exist.
Guy 1:...................Uhhhhhhhhhhhhhh...... Okay?