You say that we know that Santa does not exist. I say, using your own logical framework, that you need to prove this claim to the millions of children who fervently believe in Him. The presumptions of Santa deniers! Harrumph!

You contend that you are putting "...the idea of the presumption of atheism on trial." Is it the presumption, or the idea of the presumption? Problem is, atheism is not a presumption, and it is not founded on a presumption. Atheism is nothing more than a label, assigned to people who do not believe in a supernatural deity. And they believe that the Gods that theists believe in do not exist. They do not come to these conclusion by presumption, but by analysis of the evidence - particularly the lack thereof in this case, and a logical analysis of the claims made by theists. You want to believe that claims of atheists are just presumptions, but that is just presumption on your part.