I'm going to explain why the argument is a fallacy to begin with and from there I'm going to point out why most atheists far from being agnostics are in fact gnostic atheists because they know that god claims are false.
I'm also going to prove that all but the most rabid fundamentalists are agnostic.
Lets start with an example of the "you're not really an atheist" argument:
Theist: Have you heard the word of [insert deity]?And so on.
Atheist: I'm an atheist.
Theist: But you can't prove a negative can you? you cant be totally certain there is no god?
Atheist: Well no, not totally certain, but I think its a pretty safe bet.
Theist: Ok, but that's not the same as absolute knowledge is it?
Atheist: No it's not absolute knowledge I never claimed that.
Theist: So if you cant say for certain that you know wether god exists or not then you're just an agnostic and not an atheist!
Atheist: Well... err...
There are several problems with this argument, firstly while gnosticism is concerned with knowledge, atheism is concerned with belief, they are not mutually contradictory. Thus it is possible to be both.
Remember that atheism is the rejection of a theistic claim but does not necessarily extend to the anti-theist position of accepting the claim that god does not exist.
The second problem is the assumption that because you cannot be certain one way or the other of the existence of god that you must be agnostic. This is true only when "god" is left totally undefined, if you include a clear definition of god, such as "god as literally defined by the king james bible" then you now have sufficient inconsistencies and contradictions to say that you have knowledge that that specific god does not exist... and in fact you'll find that many (non-fundamentalist) christians would be forced to agree.
The biggest error here is the old "you cant prove a negative" chestnut which often gets bandied around.
Of course you can prove a negative! someone claims they have a real, live, ordinary 5 tonne, actual solid african elephant in their pocket, and you reach in - BOOM - Negative proved!!!
I can prove the nonexistence of a huge number of things, indeed anything with testable qualities. If your child is scared to go to bed because there is a werewolf in the closet... you open the closet to prove the negative, full stop, end of argument. In fact the only way to continue the argument (and children do this all the time) is to change what they mean by werewolf - "this one is special and can turn invisible!".
What is frightening is to see how fast people of faith revert to the tactics of a little child afraid of the dark.
The bible says the universe is six to ten thousand years old? how come we see stars that are so far away their light would not have reached us yet? - Just proved a negative, thanks for playing... but then we get "No no no no... god put the light in the sky already on it's path to make it look like the universe was older!" - Excuse me? where in that funny book of yours does it say the universe was made to old?
Its such an old trick, you prove the negative by using a testable quality and they change the claim. It's called "special pleading" and it's the main tool in the religionists arsenal against rationality.
You will often hear atheists debating religionists cede the point that god cannot be disproved - This is because the most abstract concepts of god are indeed untestable. and therefore cannot be disproved. So debaters skip quickly over the argument without having to deal claims that the monster in the closet has harry potters invisible cloak.
So remember when you speak to theists:
- You can be an atheist and an agnostic
- You dont need to be agnostic about specific claims. You can test and achieve knowledge
- You can prove a negative where testable qualities exists and if they change the definition of god then they are the ones who cannot claim absolute knowledge. They are agnostic on their own claims.
Best of all, when someone tries the "you're agnostic, not atheist" argument you can just tell them they dont understand enough about either of those terms to use them in a discussion