This assumption is rooted in the elementary logical fallacy that two opposite things -- belief and disbelief -- are actually the same thing. A basic tenet of logic is that anyone making a positive claim bears the burden of proof for that claim. For example, in a court of law the lawyers for the prosecution bear the burden of proof, because they are making the positive claim that the defendant has committed a crime.
To take a skeptical position regarding an extraordinary claim for which one has not been provided with compelling evidence is not an act of faith; it is simple common sense. Here is an analogous situation: supposedly, as a Christian, you do not believe in the Roman or Aztec gods. Is it just as much an "act of faith" on your part not to believe in those gods as it was for the Romans and Aztecs to believe in them? If a man walks up to you and says he has an invisible magic elf sitting on his head, do you automatically believe his claim? If not, is it an "act of faith" on your part not to? Or are you simply responding to the claim with common sense and skepticism because the man has failed to provide you with adequate evidence for his elf? Not believing in something when you have no reason to believe in that thing is not an act of faith, it is just the smart thing to do.
Finally, one can turn to the Bible's definition of faith -- the "substance of things hoped for, the evidence of things not seen" -- to see that this is a definition that excludes disbelief. So if you still don't agree with us that atheism is not a faith, then check your Bibles.
Posted in: About atheism