Malarkey. Atheists tend to practice situational ethics, where the ethics are judged on a case-by-case basis.
I am not an atheist, but I have one inviolable ethical rule: if Dick Cheney publically supports it, it is immoral.
Cheney privately supports his daughter Mary's gay marriage, so even Darth Vader has one ethical instance.
Regards from Rosie
from case to case, there exist different opinions. So my point still stands.
Stephen Hawking says God is not necessary for the Universe to exist.
Mathematically, God is redundant.
So, therefore, morality and ethical behavior need exactly ZERO outside arbiters.
Very many adults do not rely on fairy tales, or dusty tomes written by Middle Eastern nomads, in order to practice proper behavior.
Regards from Rosie
Maybe God isn't necessary, maybe it is, so what? I contend the cosmological argument necessitates a God, and haven't heard a decent rebuttal to it.
Outside of such an arbiter, there only exists individual preference on "moral and ethical behavior", and what one can and can't get away with in our life on earth.
I am glad you are so enlightened that Christianity is "unnecessary" for you, perhaps you find pleasure in the nihilistic darkness of a godless universe, I don't, and didn't when I was an atheist. But the fact is, organized religion is responsible for the organization of the first civilization(the first societies were run by a priestly class, such as Ur) and the first legal codes, see Hammurabi's Code of Laws. Religion serves a purpose beyond whether God exists, it unifies society and provides a cohesive moral foundation, it also provides necessary tradition and continuity for a society from generation to generation.
Your moral code, whether you like it or not, in Western society, is informed by Christian ethic, they didn't just come out of no where.