Chapter 2 undertakes to provide a critical narrative of the development and direction of international law as it was characterised by Catholic preoccupations from the Medieval and Early Modern era. Chapter 2 surveys the theological and philosophical contribution to the structure of premodern international law by Augustine, Thomas Aquinas, Francisco de Vitoria and Robert Bellarmine. They assisted the Catholic Church at crucial moments in its history and left an enduring legacy, which could broadly be described as a foundation for a Catholic cosmopolitan approach to international law. This process is highlighted and contrasted with the way early-modern international law was constructed. The structure of international law that emerged during the eventful sixteenth and seventeenth centuries led to Catholic political and legal thought becoming a minor key in the development of a secularised public international law in the nineteenth century. This distinctively different approach to sovereignty and international order provides an opportunity to examine the peripheral place of religion, particularly Catholicism, in the structure of nineteenth-century international law.