Atheism and theism address the question of belief. Do you believe the claim that a god exists, or that a particular god exists? However, there are secondary questions, which addresses knowledge. Do you know a god exists? Do you know a god doesn't exist? Both the theist and the atheist may make further claims of knowledge. By default, they are not making such claims. But any particular atheist may decide that they do claim to know that no gods exist. This is a claim of their own, seperate from the answer to the question of belief, and is about knowledge. If they make this claim, then they would be called a gnostic atheist. If they make no such claim, they are an agnostic atheist. On the whole, most atheists tend to identify as agnostic, to allow for the fact that there is the possibility of being convinced if further evidence is presented. Some may identify as agnostic because they think the answer is unknowable; it is not possible to ever have a meaningful opinion about it as it is defined in such a way as to be beyond our ability to ever examine.

 

The questions of knowledge also apply to theists. They may believe that a god exists, but do not claim to have the knowledge that one does indeed exist. This would be an agnostic theist. One who also makes the claim that they know a god exists is a gnostic theist.

 

This gives four categories: gnostic theist, agnostic theist, agnostic atheist, gnostic atheist.

 

Agnostic / gnostic atheism are sometimes referred to as soft / hard atheism, or weak / strong atheism. 

 

Of course, this brings up the question of what it means to "know" something. Are we talking about 100% certainty? Or are we talking about being sure beyond any reasonable doubt? There is no hard and fast definition here. But I would err on the side of 100% certainty when it comes to this issue. If you're not 100% certain, then you are agnostic. It could be argued that the only way to "know" a god doesn't exist is if you can prove it to be a logical impossibility. 

 

So to recap, atheism/theism answers the question of belief, while agnosticism/gnosticism answers the question of knowledge. To say you are just "agnostic" is to not have answered the question of belief. It is pretty clear to me that anyone who doesn't give the simple answer that they believe that a god exist is an atheist. If you are "on the fence" then you are an atheist. You have not accepted the claim. Whether or not you decide to label yourself as such is up to you!

 

I should point out that definitions of these words vary and some people object to them being used at all. If they are relevant to your discussion, then it is best to make sure you both agree on whatever definitions you are going to use.