I've often thought about what may have happened there. Recently I found an answer that is beginning to make sense as I ponder it. Philip Jenkins, author of The Lost History of Christianity: The Thousand -Year Golden Age of the Church in Middle East, Africa, and Asia - and How it Died(How's that for a title?!), answers our question in a post called The Other Side of Church Growth. You can read the entire interview here. The first 3 questions get to heart of the matter.
What causes church death?
In no case that I can see does a church simply fade away through indifference. What kills a church is persecution. What kills a church is armed force, usually in the interest of another religion or an antireligious ideology, and sometimes that may mean the destruction or removal of a particular ethnic community that practices Christianity. So churches die by force. They are killed.
But what about the old saying, "The blood of the martyrs is the seed of the church"?
That was said by Tertullian, who came from the church in North Africa, where the church vanished. If you were to look at the healthiest part of Christianity right around the year 400 or 500, you might well look at North Africa, roughly what we call Tunisia and Algeria. It was the land of Augustine. Then the Arabs, the Muslims, arrive. They conquer Carthage in a.d. 698, and 100 years later—I don't say there were no Christians there, but there certainly was only a tiny, tiny number. That church dies.
Why does persecution sometimes strengthen a church and other times wipe it out?
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.
Jenkins closes with some remarks we should all consider as we think about the history of the church,
My concern is that when we write Christian history, so often it's a matter of, "Let's look at this expansion, and let's look at this growth and new opportunity." We're not really seeing the doors that are closing.