[Alex Murphy]

Interesting. Would the explicit exclusion of a religious test for holding public office in the Constitution make secularism the foundation of the American Republic?

IMO, yes. All covenants declare a Sovereign who authors its "rules", initiates the covenant relationship with other parties, and who enforces the covenant if other parties fail to keep the "rules".

What Sovereign or source of law is declared within the U.S. Constitution, except the document itself?

[Celtman]

What Sovereign or source of law is declared within the U.S. Constitution, except the document itself?

We the People of the United States, ... do ordain and establish this Constitution for the United States of America.