This article explores the cognitive struggle against “doubt” that impeded the conversion of the female aristocrat Anne de Gonzague, princesse Palatine, in seventeenth-century France. Anne's conversion came only after a life-long intellectual battle that climaxed when she held a fragment of the True Cross in the fire to test whether it could withstand the flames. The intellectual premise for her experiment with the holy relic can be found in her own écrit or conversion narrative. Analysis of this narrative demonstrates that Anne's application of philosophical skepticism and the Cartesian method—to which she was exposed in the Scientific Academy of her physician, Pierre Michon Bourdelot—to her own irreligion actually brought about her conversion to orthodoxy. Anne's “test” of faith therefore compels us to rethink the relationship between the New Philosophy and faith in the seventeenth century.