The burden of proof falls to the person making the claim. Saying that there is no proof of god is not the same as saying god does not exist. The point you are borrowing from Sagan is that we do not automatically believe claims without evidence. Explain how this translates into, there is no god? A reasonable skeptic, which most atheists are, simply says there is no evidence based reason to believe in god therefore I do not believe, but show me sufficient evidence and I will. Just as with all scientific conclusions, it comes with error bars and the understanding that new evidence can change things. Extraordinary claims require extraordinary evidence. Is it likely that we will ever see evidence of god, probably not, but no reasonable persona can say it's impossible.