Long before today’s sometimes rancorous debates about faith and science, European theologians struggled to reconcile the two. During the later Middle Ages, religious thinkers increasingly drew on naturalistic explanations of the world to explain supernatural phenomena. Nichole Oresme (1320 – 1382), philosopher and theologian at the University of Paris, councilor to King Charles V of France, and bishop of Lisieux, wrote a series of treatises warning that natural or medical causes caused false religious visions. His investigations, and those of his students and followers, explicitly challenged older assumptions about the relation between asceticism, holiness, and visionary experience so familiar to medieval Christians. Oresme’s influence did not lead to a suppression of visionary practice but instead transformed late medieval and early modern religious visionary experiences and explanations for them. Preachers no longer blamed demons for all religious delusions, but instead bad diet and poor health. By assimilating medical ideas about human illness and perceptual error, theologians created a new mode of Christian pious conduct, encouraging greater emotional restraint and purposeful physiological balance to guard against false visions. Along the way, they shaped developing debates about what is lost and what is gained when religious communities modernize their beliefs and practices.