From the closing years of the nineteenth century to the threshold of the twenty-first, the Catholic Church has spoken with a steady and authoritative voice on the great social questions that have shaped the modern world. This body of teaching-rooted in Scripture, the Fathers, the scholastic tradition, and the perennial magisterium-has come to be known as Catholic Social Doctrine. Yet it is not a novelty of recent centuries, nor a concession to modern political ideologies. Rather, it is the organic unfolding of the Church's mission to illuminate every dimension of human life with the light of the Gospel. The Church, as the Mystical Body of Christ, cannot remain indifferent to the conditions in which souls live, work, form families, and pursue their temporal and eternal ends. She speaks because she must: because the moral order established by God extends to the social, economic, and political spheres, and because the salvation of souls is inseparable from the right ordering of society.