Abstract: In this article, I use historical and ethnographic data to analyse the Great Repentance, a violently emotional conversion movement that swept through the Indonesian island of Nias from colonial conquest around 1915, with recurrences until the 1960s. Against rationalist and materialist explanations, I argue for a constitutive role for emotion in the conversion process. I show how the techniques and idioms of Protestant missionaries suppressed indigenous meanings and encouraged a native emphasis on ‘the speaking heart’. The existential dilemmas of modern Christians in Nias, their sense of exclusion, can be accounted for by the paradoxical ethical and affective legacy of the repentance movement. The article is a contribution to both the study of emotion in historical perspective and to the analysis of conversion.