Defining atheism

Let P be a proposition and let A be an argument for P. That is to say, A is a collection of premises that together entail P. Clearly, P entails "Probably, P." But then A is an argument for "Probably, P" as well. Now, what about the other way around? Let B be an argument for "Probably, P." In that case B is also an argument for P. After all, if argument B provides us with a good reason to believe "Probably, P", then B provides us with a good reason to believe P as well.

Hence, on evidentialism, P and "Probably, P" are epistemically on par. Any argument for "Probably, P" is an argument for P and vice versa. Someone who believes "Probably, P" is thus also committed to P. And anyone believing P should believe "Probably, P" as well.

Consider the claim that God does not exist. Given the aforementioned atheism defined as the belief that God does probably not exist cannot be epistemically distinguished from atheism defined as the belief that God does not exist. For if there are no good arguments for atheism defined as the belief that God does not exist, then there are no good arguments for atheism defined as the belief that God probably does not exist either (and vice versa). Therefore, adding the qualifier 'probably' does nothing to the epistemic tenability of atheism.