Medieval canon law recognized detrusion (detrusio in monasterium) as a sentence for women convicted of adultery. Civil law had made adultery a capital crime, so that detrusio was a milder action. This article traces the history of detrusio in canon law, especially in the thirteenth century, and treats further questions that detrusio raised. Detrusio was originally a pastoral provision, meant to provide a woman rejected by her husband for adultery an opportunity to enter religious life. But in the hands of the jurists detrusio became a coercive ecclesiastical penalty for adultery. The practice raised further concerns, for example: how the woman's property was to be treated; whether the woman sentenced to detrusio became a religious; whether a monastery should be a site of confinement for the laity; and, under what conditions a husband could take his adulterous wife back. The case was also raised of a man who accused his wife of adultery so that he could dissolve his marriage and enter a monastery.

Thomas de Cantimpré, in his Supplementum to Jacques de Vitry's Vita of Marie d'Oignies, provides us with an account of how Cardinal Ugolino dei Conti di Segni, the future Pope Gregory IX, was struggling with his faith. At this decisive moment in Ugolino's career, the illustrious preacher and bishop of Acre, Jacques de Vitry, made an appearance at the curia. To combat Ugolino's doubt with a saintly intercession, Jacques presented him with the relic of Marie d'Oignies's finger, which he kept around his neck and which had protected him on several occasions. This well-known anecdote has not yet received any comprehensive attention and this essay seeks to analyse as well as contextualize the account of Jacques's intervention. By shedding light on the role of Marie d'Oignies and her finger relic and on the meaning of the ‘spirit of blasphemy’ plaguing Ugolino, I argue that the anecdote not only gives us a glimpse of the nature of the cardinal's spiritual concerns but also reflects Thomas's efforts to promote both Jacques de Vitry's influence on Gregory IX and the reputation of Marie d'Oignies.