Can the human mind function without a concept of some kind of God?

Atheists say that they are godless but are they really? They still have morals, and aren't morals all based on a belief that there is some entity in the universe that says that the morals are significant? To be truly godless wouldn't someone have to not believe in a "good" and an "evil"? I think atheists do believe in a good and evil, just a different type of good and evil than religious ppl. I think if an atheist today did something like kill a baby, etc they would probably feel like they had committed some sort of "sin" don't you think? and then atheists who wouldn't feel bad about murder like the Nazis would still be guilt-able in some ways weren't they? they still believed in a "good" and an "evil" didn't they? So basically my question is, do we all have a belief in good and evil, but we just differ on what that good and evil are?