This chapter explores the inseparable nature of the relationship between religion—more specifically, Christianity—secularism and international law. As the history of international law itself reveals, its inauguration as a liberal profession depended on a group of men who shared a particular universal intuition and cultural agenda that mirrored their western Christian European and cosmopolitan backgrounds at the end of the nineteenth century. Thus, the chapter scrutinizes the Catholic School of Salamanca as a case study that mirrors how Christianity—Catholic missionarism more accurately — became an integral part of international law to date, focusing on how and why the Salamancans’ specific re-configuration of the public/private has become a resilient and persistent formula to this day.