In the 52nd chapter of his ''Decline and Fall of the Roman Empire,'' Edward Gibbon posed one of the great counterfactual questions of history. If the French had failed to defeat an invading Muslim army at the Battle of Poitiers in A.D. 732, would all of Western Europe have succumbed to Islam?

''Perhaps,'' speculated Gibbon with his inimitable irony, ''the interpretation of the Koran would now be taught in the schools of Oxford, and her pulpits might demonstrate to a circumcised people the sanctity and truth of the revelation of Mahomet.''

When those words were published in 1788, the idea of a Muslim Oxford could scarcely have seemed more fanciful. The last Muslim forces had been driven from Spain in 1492; the Ottoman advance through Eastern Europe had been decisively halted at the gates of Vienna in 1683. Today, however, the idea seems somewhat less risible.

..This prospect is all the more significant when considered alongside the decline of European Christianity. In the Netherlands, Britain, Germany, Sweden and Denmark today, fewer than 1 in 10 people now attend church once a month or more. Some 52 percent of Norwegians and 55 percent of Swedes say that God did not matter to them at all. While the social and sexual freedoms that matter to such societies are antithetical to Muslim fundamentalism, their religious tolerance leaves these societies weak in the face of fanaticism.