Atheism is by its very definition a statement that you are
not a theist. So before saying, "I am an atheist" I must have been confronted with the
concept of theism in order to distance myself from that particular group of theists.
(That by itself is no proof of any
theism though, just like the
concept of a unicorn does not bring
actual unicorns into existence.)
Just like words as
health only make sense when there is an concept of
sickness to begin with. If people would not become sick, we would not have a word for
non-sickness, as it would be conveyed by default whenever we used the term
human. Just as
baldness only becomes meaningful when people
have the ability to grow hair.
This should also avoid the typical counter-argument that this definition would make rocks atheists, as those lack the ability to be theists in the first place. Even though one could describe their external features as atheistic, but that's quite a trivial thing to say.
So when I as a child confronted with
theisms, such as
Greek mythology I treated those stories as fairy tales. So I realized "I was not a Christian", and I was also not a believer in Greek gods, and more and more I learned about all the religions on this planet I realized that I could not identify with anything they claimed to be true (even though not all of them were necessarily theistic, so I also may be better described as non-religious).
The most broadest definition, I can boil
deism down to, is the positive claim that one or many divine beings necessarily exist.
And I do not claim that. Hence I am not a theist. Hence I am an atheist.
It may be that
B's theism may not fall under this definition, though I do not think of it as likely or a useful thing to do.
Yet in order for me to "prove" my atheism to
B's particular religion I would ask him to tell me what
his branch of theism entails, and tell him if I agree with it. If I say: "I do not subscribe to the presented claims" it is really all the proof you can take, good reasons approach if you will, even though I may be lying and a believer anyway.
But why would I lie? There are quite a few ad-hoc rationalizations (denial of a god, being angry at a certain god, etc. pp.) and even though I cannot stop
B from raising them, it would be the point for me to stop the discussion as futile.
For now, I feel quite rational in my poly-atheism of all the theisms I have yet encountered, and am quite biased in remaining an atheist of all the theisms I have yet to encounter, due to the huge lack of evidence any religion has yet presented and the scientific discoveries concerning the underlying principle of all religions. Yet I could be wrong.
I want to point out that I also start to consider myself as an atheist retrospectively, once I decided to ascribe the term "atheist" to myself.
Just as I think of myself as always being a boy even though I only learned the difference as a small child.