Some of William's strongest views, as conveyed in his writings, were those about ethnic and religious groups. His emphatic sense of Englishness has long been noted, and more recently his concept of ‘civilization’, of which he thought the ancient Romans and modern French the greatest exemplars. Also in recent times, attention has been given to the down-side of these notions, particularly his negative treatment of Celts (Welsh, Irish, Scots and Bretons) and Jews. What little he had to say of the Byzantines is ambivalent but on the whole not untypical of his time.Of Islam, however, he had both more to say, and more of interest.

In his book Western Views of Islam in the Middle Ages R. W. Southern distinguished between an ‘Age of Ignorance’, extending to the early twelfth century, and the ‘Century of Reason and Hope’ which succeeded it. Typical of views widely current in the earlier period are those expressed in the Song of Roland, in which the Saracens are polytheists and idolaters, Mohammed being one of their several gods.However, from c.1120, notable advances were made in Western knowledge of the Islamic religion and of its prophet-founder. Among several reasons for this, the Crusades and travels associated with or facilitated by them may be singled out as especially important. So also were the increasing and relatively amicable contacts between Eastern and European scholars in frontier areas such as Spain and Sicily.

And yet William, a monk who never travelled far from England, was the Westerner who, according to Southern, first presented a reasonably accurate account of Islam and Mohammed. Remarkably, as Southern puts it, ‘[he] … was the first … to distinguish clearly between the idolatry and pagan superstitions of the Slavs and the monotheism of Islam, and to emphasize against all current popular thought that Islam held Mahomet not as God but as His prophet.’ In support of this, Southern quotes a passage from William's Gesta Regum: