This article is a study of the process of Christianization of the city of Rome during the 5th century, especially focusing on the sermons and letters of Pope Leo I. With the help of Jan Asman's cultural memory theory, it reveals that Pope Leo I's initiatives concerning Christian festivals and commemorations played a key role in the process of the Christianization of Rome. Leo I tried to replace traditional pagan festivals with various Christian commemorations of the early Church martyrs, paying special attention to the commemorations of the Apostle Peter, regarding Peter as the spiritual founder of Rome. Since the Middle Ages, this reinvention of the cultural memory of the city of Rome became the cornerstone of the "sacred city" image in the Latin Christian tradition.