Modern Catholic Social Teaching (CST) developed in an historical context that posed dramatic challenges to the institutional Church and lay faithful. The French Revolution (1789–1799), the Napoleonic era (1799–1815), and industrial revolution, and waves of succeeding uprisings in 1820, 1830, 1848, and 1870 represented radical challenges to existing political structures, such that Old Regime conceptions of the State and Church alliance of Throne and Altar were no longer tenable. The emergence of the modern secular state in traditionally Catholic lands often included the suppression of religious orders, charitable and educational institutions, and control over clergy and hierarchy. The Church struggled in this revolutionary age of ideology – torn between laissez-faire liberalism and revolutionary socialism. The Catholic movement, primarily of laity, grew in response to these challenges, under the rubrics of the “Religious Question” of freedom of religion, and Church–State relations generally, and of the “Social Question” or how to address the growing number of rootless and impoverished industrial workers in an increasingly secularized political and cultural environment. The revival of neoscholastic philosophy of society became the paradigm through which Pius IX and especially Leo XIII were able to engage modernity on evangelical and natural law foundations.

Aquinas did not speak of “social” teaching, but did synthesize the teaching of the prophets, the Lord, the Fathers, and sound philosophy concerning social matters and responsibilities. He would have regarded the principles of CST as part of the Church’s doctrine of faith and morality (de fide et moribus), insofar as morality – the living out of that faith which consists in true beliefs about the Creator – embodies the principles, precepts, and virtue(s) of justice. For among the cardinal virtues, justice is the one bearing on those of our choices that relate to or impact on other persons, especially persons with whom in one way or another we are associated. And Aquinas’s treatment of justice, mainly but not only in his Summa Theologiae, is very extensive and detailed. This chapter offers (1) an overview of his significance for CST, (2) a review of the appeals to his writings in Rerum novarum and some of its antecedents and successors, and (3) his contribution to some leading features of CST since then, including dignity and equality; private property and associations; “subsidiarity” and the service conception of authority and law; and “solidarity” (local and global).