This paper addresses the question of the origins of unbelief in the early modern age through the story of an uneducated man named Flaminio Fabrizi sentenced to death by the Roman Inquisition in 1591. Indeed, the trial against him that was opened in Siena and concluded in Rome offers an extraordinary insight not only into his heterodox beliefs, but also into the way Fabrizi manifested his religious dissent through his behaviour, outward appearance, and verbal provocation. The questions the paper focuses on are these: Can we talk about atheism already at the end of the 16th century? Can one speak of ‘practices of unbelief’ as a challenge to religious authority in post-Tridentine Italy and elsewhere?