Nonsense. It's just a sociological fact that the cultural defaults and status quos change over time. The ancients took for granted certain beliefs that we couldn't possibly take for granted. Our background knowledge changes and affects how we're raised as children. If Popper's account of the open society entails that modern folks are perfectly openminded, that's just a positivist (a dumb empiricist) canard.

The difference between theocratic and secular societies is that the latter have freedom of thought, which means we allow our critics to speak their mind and to challenge the status quo. That hardly means we have no status quo in the first place or that ours is identical to the one that prevailed in ancient Sumer.