In The Reform Islam Needs, James Q. Wilson tries to make the point that “in time Islam will become modern, because without religious freedom, modern government is impossible” — but he also lays out exactly why that’s not likely:
Moreover, the Muslim religion is quite different from Christianity. The Qur’an and the hadith contain a vast collection of sacred laws, which Muslims call shari’a, that regulates many details of the public as well as private lives of believers. It sets down rules governing charity, marriage, orphans, fasting, gambling, vanity, pilgrimages, infidelity, polygamy, incest, divorce, modesty, inheritances, prostitution, alcohol consumption, collecting interest, and female dress.By contrast, the Christian New Testament has rather few secular rules, and these are best remembered as a reaffirmation of the Ten Commandments as modified by the Sermon on the Mount. One can grasp the whole of Jesus’ moral teachings by recalling only two things: love God, and love your neighbor as yourself.
As Bernard Lewis has pointed out, the differences between the legal teachings of the two religions may have derived from, and were certainly reinforced by, the differences between Muhammad and Jesus. In the seventh century, Muhammad was invited to rule Medina and then, after a failed effort to conquer Mecca, finally entered that city as its ruler. He was not only a prophet but also a soldier, judge, and governor. Jesus, by contrast, was an outsider, who neither conquered nor governed anyone, and who was put to death by Roman rulers. Christianity was not recognized until Emperor Constantine adopted it, but Muhammad, in Lewis’s words, was his own Constantine.
Jesus asked Christians to distinguish between what belonged to God and what belonged to Caesar. Islam made no such distinction; to it, Allah prescribed the rules for all of life, encompassing what we now call the religious and the secular spheres. If a Christian nation fails, we look to its political and economic system for an explanation, but when a Muslim state fails, it is only because, as V. S. Naipaul put it, “men had failed the faith.” Disaster in a Christian nation leads to a search for a new political form; disaster in a Muslim one leads to a reinvigoration of the faith.
Islam’s early history is one of conquest over infidels. Thus, the early Muslims never had to “render unto Caesar” — and now, hundreds of years later, Islam still explicitly states how government should be run.