It all comes down to probability. While we cannot know for sure that a magical god creature does not exist, just like we cannot know for sure that in 30 minutes humanity will not spontaniously develop the ability to teleport to and walk on the sun, it is so incredibly unlikely that for all practical purposes, one can call it a certainty. Thus, while theoreticly one must be an agnostic, atheism is practicly defensible.
On a second level, even if a magical god creature existed, it would not necessarily define or even live up to the human concept of good - it might very well be seen as evil by our standards. What theist are implying is that out standards should be set by this (imaginary) god creature, and that if it says that nazism is OK, then it is OK. This is extremely totalitarian and a flight from human responsability for determinining our own actions, as well as a way to justify bizarre moral views which would not otherwise be agreed on by humans (for instance that sex is bad unless married).
On a second level, even if a magical god creature existed, it would not necessarily define or even live up to the human concept of good - it might very well be seen as evil by our standards. What theist are implying is that out standards should be set by this (imaginary) god creature, and that if it says that nazism is OK, then it is OK. This is extremely totalitarian and a flight from human responsability for determinining our own actions, as well as a way to justify bizarre moral views which would not otherwise be agreed on by humans (for instance that sex is bad unless married).