As the boundaries between the Lutheran and Calvinist confessions hardened towards the end of the sixteenth century, music became a prominent symbol of confessional difference. In German-speaking lands, Lutheran churches often had organs whereas Calvinist worship did not; Lutherans detested the Genevan psalm tunes that were essential to Calvinist identity. This thesis examines the close relationship of music and religion in the city of Heidelberg in the turbulent period between its first fervent Lutheran reforms (1556) and the start of the Thirty Years' War (1618). By Electoral decree, Heidelberg and its churches violently oscillated four times between Lutheran and Reformed (Calvinist) confessions between 1556 and 1618. Although each change caused confessional tension throughout the city, Heidelberg's musical spheres showed continuities as much as discontinuities both within learned circles and on the popular level. Examining the theory of confessionalisation in relation to music, my thesis challenges the theory's central premise that, in the process of building unified states and using social discipline to enhance secular power, 'the three great confessions - Catholicism, Lutheranism and Calvinism - developed into internally coherent and externally exclusive communities distinct in institutions, membership and belief' (Heinz Schilling, 1995). Case-studies examine the hymn repertory in Heidelberg, the role of music in confessionalised education, the uses of music books, and music in court ceremony. Combining hitherto unknown archival material and numerous printed sources with methods of cultural history, gender theory and theory of ritual, my thesis revives previous views about Calvinist musical culture, shows the extensive overlapping of Calvinist and Lutheran musical cultures, and explores the relationship of confessionalised music to pre-Reformation religious and political networks.