Vernacular=Christianity vs. Lingua Franca=Islam?

He said, in effect, Christianity spread rapidly in areas where literacy in local vernaculars was well established, whereas Islamization predominated in those where lingua francas were important. A very interesting observation indeed, although its applicability in areas outside Africa may be debatable. We also have to take into account that lingua francas may become mother tongues as a result of nativization and language shift.