This chapter examines militant atheism under Vladimir Lenin and Joseph Stalin, focusing on how the Bolsheviks approached religion from the revolution in 1917 until Stalin's death in 1953. Using legal and administrative regulation, extralegal repression and terror, and militant atheist propaganda, the Bolsheviks sought to build a new Communist world, remake society, and transform human nature. The chapter first provides a background on Russia's “old world” in order to understand the political, social, and cultural landscape that the Bolsheviks inherited when they seized power in October 1917. It then considers the Marxist–Leninist framework within which the Bolsheviks understood religion, the Bolsheviks' atheist propaganda and scientific enlightenment, and byt (culture of everyday life) as the final frontier in the Bolshevik Party's war against religion. It also describes the Bolshevik Party's adoption of the Stalinist religious policy, Stalin's wartime rapprochement with the Russian Orthodox Church, and his decision to abandon atheism.