Russia’s place in the sixteenth-century European Reformation has remained largely an understudied subject in the West. Indeed, most early modern scholarship rarely crosses the Carpathians or Lake Peipus. Similarly, in Russia, scholars have not yet produced a systematic study of Ivan the Terrible’s views vis-a-vis the Reformation. This article examines Ivan the Terrible’s polemic against Protestantism within the context of the regional eastern European and larger, continental Reformation. For Ivan, Protestants constituted both internal and external threats, posing grave danger to Orthodox doctrine, Russian society, and the safety of Muscovy’s borders. Theologically, in the czar’s worldview, Protestants were heretics and schismatics par excellence, whose split from apostolic succession endangered not only Roman Catholicism, but the integrity of the Orthodox church as well. Socially, Ivan thought, Protestants championed moral degeneracy, while politically they threatened global and regional security by overwhelming neighboring European states with upheavals and revolt.