Answer:
During the Middle Ages, the institution that united Europeans the most was the Church. The Catholic Church played a central role in medieval society, providing a common religious framework and a sense of unity among people across different regions and social classes. The Church's influence extended beyond religious matters and encompassed political, social, and cultural aspects of medieval life. It provided a sense of stability, moral guidance, and a shared identity for Europeans during this period. While other factors such as the common sense of Roman cultural heritage, cooperation of feudal leaders, and respect for the rule of law also had some unifying effects, the Church was the most prominent and influential unifying force in medieval Europe.