The argument that I have heard is that people who don't know the difference between right and wrong aren't lacking religion, but empathy. Although correct, there are always going to be some who do lack empathy. This also assumes that your society values empathy, which I suspect was deficient in the Nazis, the Roman Empire and the Soviet Union. Therefore, I see no guarantee that a society would value anything beyond individual or national self interest. Many would, but some would not. You can make a convoluted argument for morality, but some would find these arguments too deep and discard them.
Of course, the atrocities committed under slavery in this country, and others, showed a complete lack of empathy, and religion was often used as the justification. Sam Harris claims that good people will do good things, and bad people will do bad things, but if you want a good person to do bad things then you need religion.
I am not taking sides here. I just think that the ideas are interesting.