Which issue was the most frequent source of conflict between church and state in the Middle Ages?

The doctrine of papal supremacy was a central issue in the conflicts between church and state during the Middle Ages because it positioned the pope as the ultimate authority in spiritual matters, often at odds with secular rulers. This doctrine asserted that the pope held authority over all Christians, including kings and emperors, leading to tensions over sovereignty and governance. Monarchs frequently challenged this supremacy, trying to assert their control over ecclesiastical appointments and governance within their realms, which led to significant power struggles, such as the Investiture Controversy.

In contrast, while the role of kings in worship, the establishment of national churches, and the regulation of religious texts also generated conflict, these issues were often shaped by the broader context of papal authority. The centrality of the doctrine of papal supremacy highlights its predominant role in the ongoing disputes during the period, making it the most frequent source of tension between church and state in the Middle Ages.