One of the benefits of Adel Guindy’s new book, A Sword Over the Nile: A Brief History of the Copts Under Islamic Rule, is that it implicitly answers an important question: how and why did non-Muslim nations become Islamic? In this case, how did Egypt go from being overwhelmingly Christian in the seventh century to being overwhelmingly Muslim in the twenty-first century? To understand the significance of this question -- and because pre-Islamic Egypt’s profoundly Christian nature is often forgotten -- a brief primer is in order: Before Islam invaded, Egypt was home to some of Christendom’s earliest theological giants and...