10C Questions

When did "secular" come to mean "neutral" or "agnostic" in legal and political theory?  What good sources track this development?  What good sources explore the idea that "secular government" and "religious institutions" operate in separate spheres and possess a degree of autonomy from one another but that both owe their foundation and their allegiance to God and must operate consistently with His commands.  In other words, what good sources (historically and contemporary) argue that "secular" cannot be "agnostic" or "neutral" between theism or atheism as a matter of the foundations of government although it might be neutral in the granting of benefits or the imposition of burdens in particular cases?

Thanks, Michael