Shouldn't a lack of belief in god imply:
P(not("God exists")) = 0.5
P("God exists") = 0.5
(I am completely ignoring the very important part of defining God in the sentence as I take the question to be asking of a way to express 'not knowing' in probabilistic terms. This can be applied to any subject really.)
I am completely ignoring the very important part of defining God
That is indeed the chief problem here. I'm assuming you're talking about the prior probability which we have before looking at the evidence.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.
To any future monthly posters of SQ threads, please remember to add the "stupid_questions" tag.