EDIT: I've argued multiple times on this site that atheism refers to a class of belief systems, namely those those belief systems that do not include the belief "There exists at least one god." Many different belief systems might implement atheism--say, secular humanism, or some suitable version of Platonism. Given this understanding of what atheism means, it's not entirely clear what it would mean if atheism were true. Perhaps it would simply mean "if a belief system does not implement atheism (i.e., the system is theist), then the system necessarily contains a falsehood".
But I'm wondering if you could define 'the' Atheist belief system as being the intersection of every belief system that implements atheism. Interestingly, under this definition, 'the' Atheist belief system is equivalent to nihilism (the empty belief system). Kind of cool.