There was no systematic re-conversion, but there was (eventually) a mass expulsion of Muslims.

Hard numbers are always elusive in pre-modern history, but it seems that considerable numbers of Sicilians had converted to Islam, particularly in and around the major cities. In combination with migration from North Africa, this process created a substantial Muslim population by the time of the Norman Conquest.

The Norman counts (and then kings) were remarkably tolerant of religious diversity. Although former Christian churches that had become mosques were re-consecrated, there was no attempt to forcibly convert Muslims. The Normans preferred the "divide and rule" strategy: Greek Orthodox, Catholic, and Muslim communities were generally left to their own devices, as long as they paid their taxes and didn't cause trouble. Typically, Muslim mercenaries made up the bulk of the Norman kings' armies, while their navies were usually commanded by Greeks. King Roger famously maintained several Muslim scholars (including the historian al-Idrisi) at his court.

A few Muslim conversions to Christianity are recorded in the first years after the conquest. The Emir of Enna, for example, voluntarily converted after Count Roger gave him favorable terms of surrender. But despite some evidence for Muslim migration from Sicily (to Spain or North Africa) in this period, there is no indication that political pressure was ever applied by the Normans.

In the early thirteenth century, however, Frederick II (though personally tolerant of Islam) exiled some 20,000 Sicilian Muslims to southern Italy after a rebellion broke out among the Arab populations in the western part of island. This act seems to have critically weakened the Sicilian Muslim community.