This article examines the political role of illness in Émile Zola’s La Faute de l’abbé Mouret (The Sin of Father Mouret) (1875) in articulating the difference between a religious and a secular body. Published in the early French Third Republic (1870–1940), this novel shows the Zolian body as the nexus upon which religious and republican discourses compete. Using Paul Ricœur’s theory on Christianity’s original sin, this article compares Mouret’s sickness with physical evil and illustrates how Zola redeploys the traditional religious symbols of the heart, the blood, and the Word to the secular realm. It will show that original sin is a Christian myth inscribed on the body, and that Zola’s reformulation of a core religious doctrine and its supporting framework can and must be dismantled for the fledgling secular Third Republic. The article shows an attempt by Zola to forge a republican self, and thereby offers a new perspective on the nature of the Zolian body which merits further study under the field of Medical Humanities. Through the construction of the religious body, the article also contributes to wider critical discussion on mythology in Zola’s work.