It's well worth a read, and you can find it here.
Here's an excerpt:
CT: Why does persecution sometimes strengthen a church and other times wipe it out?
PJ: The difference is how far the church establishes itself among the mass of people and doesn't just become the church of a particular segment, a class or ethnic group. In North Africa, it's basically the church of Romans and Latin-speakers, as opposed to the church of peasants, with whom the Romans don't have much connection. When the Romans go, Christianity goes with them.
But Christianity establishes itself very early as a religion of the ordinary, everyday people in Egypt as things get translated into Coptic. As a result, after almost 1,400 years under Muslim rule, there is still a thriving Coptic church that represents [perhaps] 10 percent of the Egyptian people—which I would personally put forward as the greatest example of Christian survival in history