Ellul has an interesting chapter on the influence of Islam on Christianity. Like much else in this book, I think Ellul is something of a perfectionist who is overly critical, only seeing the failures of the Church, but this can of course also be helpful in pointing out real error.
Here he says that the rise of Canon Law comes to the West through Islam in the East. “I am inclined to think, for example, that the law of serfdom is a Western imitation of the Muslim dhimmi. Religious law is also important. I am convinced that some parts of canon law have their origin in Arab law.” (97)
Ellul goes further by suggesting that it’s a philosophical problem. Thoma Aquinas not only gave us a great synthesis of Aristotle and classical philosophy, he did it as a result of Islam. “We speak of Greek philosophy and Christian theology. But this Greek philosophy was faithfully transmitted by Arab interpreters. It was by way of Arab-Muslim thinking that the problem came to be addressed at this time.” (97)
Ellul draws a straight line from Muslim unitarian monotheism and legalism to what he calls the “juridicizing of Christendom,” pressuring theology into purely legal categories (99). And once religion has taken on this political role, it is not surprising to see it turn violent. Ellul says that it cannot be considered an accident that shortly after Christians come in contact with militant Islam, the crusades emerge on the scene as plausible options. “One fact, however, is a radical one, namely, that the Crusade is an imitation of the jihad. Thus the Crusade includes a guarantee of salvation. The one who dies in a holy war goes straight to Paradise, and the same applies to the one who takes part in a crusade. This is no coincidence; it is an exact equivalent.” (103)