The study of moral questions and the foundations of morality in the light of Christian belief. From earliest times Christian thinkers were concerned with questions of morality, but moral theology began to emerge as a discipline independent of dogmatic theology only at the end of the 16th cent. Since the early 19th cent. Protestants have generally preferred the title ‘Christian ethics’ for the discipline of Christian moral enquiry.
The ‘Didache’ contains perhaps the earliest Christian treatise on moral theology in its teaching of the Two Ways. With the conversion of large numbers of pagans in the 4th cent., strict moral teaching became urgent. In the W. St Augustine's adaptation of classical and Neoplatonist thought to Christian theological purposes was the dominant patristic influence on medieval ethical thought; he established charity (or love) as the fundamental principle of Christian morality from which other values flow. With the revival of Aristotelianism in the 13th cent. St Thomas Aquinas linked moral theology to natural law, the natural and supernatural virtues, and the gifts of the Holy Spirit. At the same time, precursors of the modern casuistry made their appearance with Raymond de Peñafort's Summa de Poenitentia (c.1225).
At the time of the Counter-Reformation the RC Church needed to respond to the Protestant emphasis on grace. The period was marked by the growth of, and controversies over, different systems of casuistry, especially Probabilism, and by numerous manuals on moral theology. This development was fostered by the increased frequency of sacramental confession. The most renowned moral theologian in modern times is St Alphonsus Liguori. Against the harsher Probabiliorist method, then common in France and Italy, his Theologia Moralis (1753–5) established the milder Equiprobabilism. In the 20th cent. RC moral theologians sought to give greater prominence to biblical authority, to the role of moral theology in providing positive guidance in Christian living rather than instructions to confessors about minimum standards of obligation, to the social dimensions of human existence, and to ecumenical dialogue.
Protestants have tended to dissociate themselves from attempts to produce systems of duties binding on all Christians, arguing that good works are a free response to the completed work of justification in Christ. J. Calvin gave more weight to the directive use of moral law than M. Luther, and in 17th-cent. England both Puritans and High Church people were interested in moral theology, but in the 18th cent. Protestant thinkers looked increasingly to moral philosophy for guidance. In the 20th cent. K. Barth's revolt against liberal theology helped to reinstate in Europe a distinctively theological conception of ethics, while in the United States of America R. Niebuhr's attack on the ‘Social Gospel’ movement employed an Augustinian awareness of sin to criticize the dominant optimism. Distinctions between RC and Protestant moral theology have become less sharp, and in all Churches cultural and technological change has set the agenda for many debates in moral theology, e.g. concerning bioethics, social and economic justice, and the morality of modern war.