I have seen this venn diagram or variations on it many times before and I have always found it unnecessarily over complicated.
Theists believes in the existence of god(s) regardless of evidence
Atheists do not find sufficient evidence to do so.

Every thing else is either a personal choice or an inability to be decisive. None which ultimately needs to be labeled.

The most simple test I can think of is if your child is dying is your first inclination to call a doctor or a priest to save then?
If a doctor you are instinctively atheist, if a priest you are an idiot.