Cuius Regio, Eius Religio: Political Origins of Religious Homogeneity in Western Europe

In this paper, it is argued that the relative religious homogeneity that is still observed in Western (Catholic and Protestant) polities worldwide resulted from a unique security dilemma that the European monarchs faced during the Middle Ages. The existence of a powerful extraterritorial religious authority with punitive powers (e.g., excommunication, interdict, Crusades), the Papacy, which lobbied for religious sectarian homogeneity in an environment of fierce interstate competition without a hegemonic power, compelled the monarchs to convert, expel, or kill their non-Catholic subjects. The interaction between the Papacy and the monarchs resulted in the construction of religious homogeneity, approximating a religious sectarian monopoly. The paper demonstrates this mechanism by focusing on the interaction between the Papacy and the local monarchs in the eradication of large non-Catholic populations in three critical Western European polities: Muslims, Jews, and Orthodox Christians in Sicily and southern Italy (1091-1300); Cathars in France (1209-1229); Jews and Muslims in Spain (1085-1492). As a result, Western European polities that were about to colonize the rest of the world had populations that were almost entirely Western Christian (Catholic or Protestant) by the 16^th century. Alternative hypotheses based on geography, modern nationalism, or Christian intolerance fail to explain this outcome. Moreover, even after decades of mass immigration to Western countries, the largest non-Christian minority, Muslims, only made up 1% of the population on average in Catholic and Protestant countries as of 2000. This outcome has significant consequences that cannot be understated, and it has not been studied as a puzzle before.