In talking of the Christian world, in English and, I suppose, in all the other languages of the Christian world we use two terms: Christianity and Christendom. Christianity means a religion, in the strict sense of that word, a system of belief and worship and some clerical or ecclesiastical organization to go with it. If we say Christendom, we mean the entire civilization that grew up under the aegis of that religion, but also contains many elements that are not part of that religion, many elements that are even hostile to that religion. Let me give one simple example. No one could seriously assert that Hitler and the Nazis came out of Christianity. No one could seriously dispute that they came out of Christendom. In talking of Islam, we use the same word in both senses, and this gives rise to considerable confusion and misunderstanding. There are many things that are described as part of Islam, which are indeed part of Islam, if we take the word as the equivalent of Christendom, but are very much not part of Islam are even alien or hostile to Islam if we take the word Islam as the equivalent of Christianity. I think this is a very important point, which one should bear in mind.
Bernard Lewis