That's a very secular western view of religion. The distinction between law and religion is one seems important to people familiar with Christianity, because that religion has historically existed alongside a state with a separate system of laws (either English common law or some derivative of Roman civil law). It's a distinction that makes a lot less sense in Islam, which has historically not existed alongside a separate state, but has been an integral foundation for the state.
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.
That's a very secular western view of religion. The distinction between law and religion is one seems important to people familiar with Christianity, because that religion has historically existed alongside a state with a separate system of laws (either English common law or some derivative of Roman civil law). It's a distinction that makes a lot less sense in Islam, which has historically not existed alongside a separate state, but has been an integral foundation for the state.