The pontificate of Leo XIII (1878-1903) marks a turning point in the relationship between the Catholic Church and modernity. On the political front, the encyclicals Diuturnum (1881) and Libertas (1888) opened a gradual acknowledgment of representative government, while grounding political authority in a transcendent framework; on the social front, Rerum Novarum (1891) offered the first systematic response of Catholic teaching to the labour question, condemning the excesses of industrial capitalism, rejecting socialist solutions, and proposing a positive agenda - workers' associations, mutual aid, state intervention - that would shape Catholic social and legal thought for generations. The lecture examines these two strands as expressions of a coherent magisterium aimed at securing an active Catholic presence within liberal society.