Christianity arose in a position of extreme weakness relative to the state, and remained in this position for centuries. Moreover, despite unambiguously affirming the state’s legitimacy (as in chapter 13 of St. Paul’s letter to the Romans, for example), the early Church was subject to relentless persecution by the state. These contingent historical factors might have been enough to guarantee that Christianity would come to regard Church and state as having fundamentally different missions. But Christian doctrine entails that in any case. Though the Jews of his day hoped for a political Messiah who would take up arms and free them from Roman domination, Christ famously declared: “My kingdom is not of this world” (John 18:36). He also commanded: “Render to Caesar the things that are Caesar's, and to God the things that are God's” (Mark 12:17), indicating that the political and religious orders are distinct. The letter to the Hebrews teaches that the patriarchs of old -- who are models for the Christian to follow -- “were strangers and foreigners on the earth” who “desire[d] a better country, that is, a heavenly one” and that God has indeed “prepared a city for them” (Hebrews 11: 13, 16). The letter to the Philippians says that “our commonwealth is in heaven” (3:20). St. Augustine distinguished between the “earthly city” and the City of God. And so forth.
There has from the very beginning of Christian teaching, then, been a clear distinction between the religious and the political, between the sacred and the secular, between Church and state. (Notice that I said a “distinction” between Church and state; I did not say a “separation,” which is a very different idea, to which I will return below.) The distinction would eventually come to be given a theoretical articulation in terms of a further distinction between the natural order and the supernatural order.
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The liberal tradition essentially begins with Hobbes and Locke. What it inherits and preserves from Christianity is the idea that Church and state are distinct and have different missions, and that the state’s mission is something which can be determined from natural law or unaided reason rather than special divine revelation. But it departs from the Christian tradition in several crucial ways. First, it introduces a highly desiccated notion of the “natural” and thus a highly desiccated notion of reason and natural law. Second, it does not regard the state as natural but as entirely man-made, though it still regards the state as rational insofar as it takes us to have good rational grounds for creating it. Third, it tends to regard revelation, and indeed religion in general, not only as distinct from the order of natural or unaided reason, but as positively at odds with reason. Fourth, for that reason it regards the Church as something which is not only distinct from the state but which ought always and in principle to be kept rigorously separate from the state, or indeed even subordinate to the state. Fifth, given its desiccated notion of “nature” and tendency to pit religion in general against reason, it also has a tendency to exclude even the generic theism of natural theology from the political order. In short, from Christianity, liberalism “chooses” or “takes” the natural and secular, radically redefines them, and excludes the supernatural and the sacred. And in that sense it is a kind of “heresy.”
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Muhammad’s program was religious, to be sure, but by no means merely religious. Or to be more precise, he did not regard the cultural, moral, legal, economic, military, and political spheres as something distinct from the religious sphere, to which religion may or may not be applied. They were all just parts of one sphere, the religious sphere, from the get go. Muhammad was prophet, statesman, legislator, general, and cultural and moral exemplar, all rolled into one. And Islam was, accordingly, not merely a program of religious reform, but a program of complete social and political reform, every aspect of which -- not merely the theological aspect -- was grounded in the revelation Muhammad claimed to have received from God.
Not that everyone got with the program, at least not initially. Muhammad faced opposition, so much so that he famously had to flee from Mecca to Medina. But this opposition did not succeed for long, and soon the entirety of Arabia, as well as North Africa, the Levant, Mesopotamia and Persia, knew the power of Islam -- its temporal power, its political and in particular its military power, no less than its spiritual power. Muhammad’s kingdom, unlike Christ’s, was from the start very definitely of this world, and his servants certainly fought. And unlike the Church during the first centuries of Christianity, Islam was not in a weak position relative to the state. That is not because Islam controlled the state. It is because Islam was the state. The caliphate was not a secular power over which Islam had acquired an influence, not a state to which a distinct Islamic “Church” had been annexed. It was “Church” and state in one. Or rather, it was all just Islam, because there is in Islam no such thing in the first place as the notion of a “Church” understood as a purely religious institution, which might be distinguished from some other institution called “the state,” to which it may or may not be fused.
It is a fundamental error, then, to try to understand Islam or its history on the model of the relationship between Church and state in Christian history. To do so -- and to suggest on the basis of this analogy that the separation between Church and state that liberalism achieved might be duplicated in the Muslim context -- is simply to ignore the actual history of Islam (and, ironically, to impose alien Western categories on Islam in the very act of trying to defend it against its Western critics). It is particularly absurd to propose, as some Western liberals do, a “separation of mosque and state,” as if the notion of the mosque were the Islamic equivalent of the notion of the Church. For one thing, the word “church” is ambiguous in English. It can mean a certain kind of building, or it can mean the Church as an institution, distinct from other institutions like the state, the family, a business corporation, etc. There is no parallel ambiguity in the word “mosque.” It’s just a building. For another thing, it is not a building devoted merely to what Westerners think of as purely religious affairs. Rather, it is a place wherein the Muslim preacher might also just as well discuss politics, culture, economics, etc. -- because, again, these are all just as much a part of the concerns of Islam as purely religious matters are. The idea of a “separation of mosque and state” is therefore a muddle.