[bawolff]

Well you can of course take anything you want as a first principle, this is kind of an odd choice. Typically people take as first principles foundational things they believe to be true but know they can't prove. Things like, our memory reflects things that happened in the past or other people exist as real people and aren't just complex NPC.

Most atheists believe that there is no reasonable evidence for a deity, thus it makes no sense to believe in one (or some varation of that). In particular they believe this conclusion is rational and can be arrived at through reasoning. Taking "there is no deities" as a first principle, is in essense taking a view about religion on just "faith". My understanding is that to most atheists, this sort of appeal to faith is an anathema.