I must push back on your thesis statement here. There is no 'first tenet' of Atheism in this sense, considering there is no Church of Atheism, nor is there a 'holy' doctrine which governs all Atheist thought.

Futhermore, most Atheists would stop short of making the claim 'god does not exist' as a statement of fact, but would only go so far as to say 'I will act as if god does not exist because the evidence thusfar presented for a god existing does not meet the burden of proof necessary to positively claim as fact that god exists.'