Mater et Magistra (1961) expanded the tradition’s scope to address international development and the relationship between wealthy and poor nations. It remains one of the most important documents in the history of Catholic engagement with the modern world. His phrase “development is the new name for peace” entered the permanent vocabulary of Catholic social thought. And then came another LeoOn May 15, 2026 — the 135th anniversary of Rerum Novarum — Pope Leo XIV published Magnifica Humanitas. Leo XIV chose his papal name in honor of Leo XIII, and the choice of May 15 for his first encyclical placed him explicitly in the line of succession of the tradition his predecessor had begun.