On 15 January 1875, the Belgian economist Émile de Laveleye published an article that shook the European world of publicists, essayists, economists, and political scientists. The text, which appeared in the Revue de Belgique, was entitled “Le Protestantisme & le Catholicisme dans leurs rapports avec la liberté et la prospérité des peuples. Étude d’économie sociale.” It argued that Europe found itself at a critical juncture of its history, while affirming at the hand of a rich argument deserving of analysis and interpretation that conversion to Protestantism represented the only way to avoid falling into some form of decline. As an internationally renowned scholar, whose correspondence network included the likes of William Gladstone, Edgar Quinet, and John Stuart Mill, Laveleye was one of the inspirational sources for Max Weber, who in 1904-1905 published his well-known The Protestant Ethic and the Spirit of Capitalism. The main aim of this article is to study Laveleye’s important reflection from 1875, which saw translation into nearly ten different languages, and to see what it says about the issues of the time. As a useful source for the history of nineteenth-century Protestantism and of the Franco-Prussian War of 1870-1871, it at the same time testifies to the infiltration of intellectual movements coming from the United States since the 1850s (Unitarianism) and to an attempt to revitalize the magisterium in the wake of Edgar Quinet. Above all, however, Layeleye’s work is an intellectual and political act in the history of the founding of the Third Republic at a time when it adopted constitutional laws, and where Laveleye called for a republican spirit served by the Protestant faith, whose decline he feared under the attacks it faced from the side of the church and the claimants to the restoration of the monarchy.