"Civil law cannot be separated from Biblical law, for the Biblical doctrine of law includes all law, civil, ecclesiastical, societal, familial, and all other forms of law. . . . Law is in every culture religious in origin. . . . in any culture the source of law is the god of that society. . . . in any society, any change of law is an explicit or implicit change of religion. . . . no disestablishment of religion as such is possible in any society. . . . there can be no tolerance in a law-system for another religion." pp. 4-5.

There's not a lot of sophisticated thought behind this logic.  Rushdoony simply re-asserts political thought that was typical before the enlightenment.  In those days most everybody believed that governments derived their authority from the gods and that rulers ruled by divine sanction and decree.   All he did was to transpose ancient clashes of civilizations from the realm of religion to the realm of law and