Throughout his work Johann Christian August Heinroth regarded sin to be the cause of mental illness. The present two-part paper investigates what exactly Heinroth understood by sin. Based on a thorough analysis of his own texts, this study shows that on the one hand Heinroth referred to sin in a Christian-Protestant sense. On the other, however, a moral-ethical code of conduct was also involved. Thus, Heinroth did not regard sin as a singular event, but rather as a life conducted in a wrong way for years or even decades, by which he meant a steady striving towards earthly, bodily satisfaction.