“Every society other the the post-modern western world believed in objective moral truths.”
Not true. Greco-Roman paganism, and most other early belief systems, had no concept of a central moral truth.
They were based instead on the idea that the gods were venal and capricious beings and divine judgement was avoided only by not angering them.
Even Zeus used a balance to tell him what is fair and just, which means even the Pagan Greeks believed in an objective measure of justice.