I wouldn't say that Christianity has "historically" been particularly well factored from a separate system of laws, unless you mean to start history with the Enlightenment. The seat of power in Constantinople for centuries was a shifting alliance of Emperors and Patriarchs, and the law that was enforced was not really separated into secular and church law, but more like negotiated spheres of influence of each leader's representatives. The same goes for most of Western Europe's history, which involves a very long shadow cast by the Pope, and an extensive role for the Catholic Church in comprehensively ordering society and the operation of courts—in many places and eras there was essentially no distinction between canon law and civil law. This is because, exactly as you say, religion has traditionally been considered an integral foundation for the state in Christian countries.