Natural law does not depend upon religious orthodoxy, though it is often associated with it. One can defend natural rights from the perspective of being inherent in our existence as rational beings rather than Sky-King's (Sky-Kingses' for the polytheistic?) decree.

Further, large numbers of people with at least some theistic leanings were/are nevertheless adherents to legal positivism, which explains how you can have Catholics or Lutherans that were okay with the Nuremberg Laws, for example.