Knight said:History would prove you wrong.
But moreover, if you are REALLY a moral relativist you should argue against such "common law" as that would defeat the relative position of the "law".
How so? If sexual assault and homicide have ever been legalized lemme know...
Culture and civilization has always recognized certain actions are beyond the pale and illegalized them. Even the most decadent of societies punished certain crimes. A religion's law word isn't necessary to abolish murder or illegalize rape.