This dissertation traces adapted forms of the medieval mystery cycle tradition within different transnational moments of social, political, and cultural crisis. In redirecting the spiritually didactic aims of medieval performance, the modern mysteries that constitute this project illuminate how medieval theatre functions as an historical imaginary for the transformative potential of performance.This project investigates three twentieth-century adaptations of the medieval mystery cycle tradition: Alexander Scriabin’s unfinished multi-genre performance, Mysterium (c. 1910); Jean Paul Sartre’s first play, Bariona (1940); and a South African production of the Chester Mystery Cycle, Yiimimangaliso (2000). Chapter 2 demonstrates how Mysterium sought to enact a distinctly medieval imaginary of spiritual unity epitomized by the Russian religious value of sobornost.’ In analyzing its Russian Symbolist aesthetics, I argue that the Mysterium was designed phenomenologically to enact social transformation on the eve of the Soviet revolution through "affective atmosphere.” Chapter 3 discusses Jean-Paul Sartre's relatively unknown play Bariona as an adaptation of the medieval French nativity play tradition produced during World War II. This chapter situates Bariona within the longstanding tradition of French medievalist performance as a contested political site within the national consciousness. By analyzing its carceral creation in a POW camp, I argue that Bariona enacted a spiritual and liberatory efficacy through the phenomenology of the gaze. Chapter 4 discusses Yiimimangaliso, a South African adaptation of the Middle English Chester Mystery Cycle, as form of post-colonial syncretic theatre. Staged in the wake of apartheid, Yiimimangaliso's disparate domestic and international reception demonstrates how the "unmodern" is exoticized and consumed in both medieval and racialized forms while enacting a new notions of nationhood. Though stemming from vastly different genealogies, these performances converge on their invocation of the medieval mystery as a performed imaginary of cultural and national unity during times of national rupture. By tracing their respective generation and reception, this project argues for the “mystery” as a theatrical modality that seeks to interpellate spectators into new, transformative subjectivities that disrupt binaries between secular and sacred during moments of social, political, and cultural change.