Among Protestant readers, Jesuit Robert Southwell's Saint Peter's Complaint was a surprise best seller. First published in 1595, one month after his martyrdom, South-well's poem underwent thirteen editions by 1634. This essay examines the grounds of that popularity by exploring how Southwell's portrayal of Peter's repentance might have appealed to readers across confessional boundaries. It contends that Southwell's familiarity with the landscape of affective piety in post-Reformation England allowed him to make a self-conscious entreaty to Elizabethan readers caught in the crossfire of reform. It first examines the affective contours of repentance depicted in Protestant devotional works, many of which were rooted in Calvinist movements of sorrowfulness, mortification of the flesh, and vivification of the spirit. Southwell, it argues, distends these movements in Saint Peter's Complaint in a way that echoes not only Protestant conceptions of penitent weeping but also Protestant pastoral practices. This cross-confessional deployment of religious affects highlights the overlapping practices of piety within Protestant and Catholic devotion. It also intimates the methods by which South-well might have utilized devotional poetry in service to evangelism.