College-level courses devoted to Renaissance culture typically put a premium on incorporating primary sources and artifacts of a literary, art-historical, and historical nature. Yet the monuments of contemporaneous music continue to be marginalized as instructional resources, even though they are fully as worthy both from an aesthetic and from a historical standpoint. This study attempts to address that problem by invoking the tradition of early polyphonic masses on L’homme armé – a secular tune used as a unifying melody (cantus firmus) throughout settings of the five-movement liturgical cycle. Beginning by explaining the origins and significance of the putative monophonic tune, the paper then details how a series of composers utilized the song in interestingly varied ways in various mass settings. Subsequently it sketches out a context for mysticism in the liturgical-musical tradition of L’homme armé, and points to some compelling parallels with the contemporaneous art of panel painting, specifically as represented in the works of Rogier van der Weyden.