The history of Italian Protestantism in the nineteenth century has been often written using sources belonging exclusively to the Protestant Churches, sometimes not taking into account the documents of the Roman Congregations. Thanks to the new availability of archival material (for example, thanks to the opening of the Archive of the Congregation for the Doctrine of the Faith), the article elucidates the role played by the Holy Office in the control of Protestant preaching in Rome and the Lazio in the 1870s. Via the study of reports and abjurations, the essay focuses on the Inquisition's strategies of control, on its networks, and on its handling anti-Protestant fears, in an attempt to bring converts back to Catholicism. The article shows the major fears of the Curia at the time: those that led to identify Protestantism with Mazzinianism, Freemasonry, and social activism. On the other hand, the essay sheds new light on the social profiles of those who joined the new Protestant churches.