Expertise not required for entry
Not believing there is a god should be enough (enough for atheism, enough for being an atheist). We shouldn’t have to sign up to more. We don’t have time to figure out all the things that we think don’t exist. We can just not think they exist, and let it go at that – or we can not think they exist and then go on to think they don’t exist, if we want to and have time, but that’s extra. Just not thinking so is the minimum needed for entry, or at least it should be.
There’s no sense in believing things exist for no reason – so we don’t (if we have sense) – and for atheists ‘god’ is one of those things. That’s important. The negative matters more than the affirmative.
The minimal definition matters because it has to do with reasons. We don’t believe because we see no good reason to believe – we know of no evidence that god exists. Believing that god doesn’t exist requires some as it were expertise – and like theism, atheism is a public, non-expert view. You can have more detailed or engaged or ‘expert’ atheism, but that shouldn’t be the main definition, because everyone should be able to Just Say No as easily as everyone is able to say yes.