In recent years, historians have questioned the notion that belief was central to the Reformation’s success, arguing rather for a variety of social, political, economic, and psychological forces. This study examines one of the intra-Lutheran doctrinal debates, the Flacian controversy over original sin, as means to analyze lay religiosity in the late Reformation. It focuses on the German territory of Mansfeld, where the conflict had miners brawling in the streets, and where a wealth of sources from the laity have survived. This extraordinary evidence demonstrates that although diverse forces were at work, by the late sixteenth century many commoners had developed a complex understanding of Lutheran doctrines, and these beliefs had become informing factors in the laity’s lives.