This collective monograph has three objectives: to clarify the form of paganism‘s survival in the Christian society of the Czech and Polish kingdoms in the Middle Ages; to examine the coexistence of paganism and Christianity in Bohemia and Poland between the 11th and 15th centuries; to shed new light on the processes of external and internal Christianization, which significantly shaped late medieval piety. It seems clear that significant and in many ways unique religious processes took place in medieval Central Europe, conditioned by the need to cope with paganism and the delayed adoption of Western European models of church administration and religious life. In the 15th century, however, Christianized society in the Bohemian and Polish kingdoms began to ask questions that went beyond the intellectual horizons of Western Europe. In the Czech milieu, this process resulted in the Hussite Reformation, a reformation before the Reformation, while in the Polish milieu it gave rise to the concept of unlimited religious tolerance towards pagans derived from natural law and embodied om the, doctrine of ius gentium, as well as to the development of ideas about the uniqueness of Polish Christianity as a bulwark against heresy (Hussite) and Islam.