An excellent overview of important Catholic Contributions to Western Law is provided by Michael P. Foley on the CERC website, can be found here. It is worth reading in full, but in just a few short paragraphs the author introduces many of the most important principles guiding the Church's influence on Law. Most of these ideas are covered more fully in Thomas Wood's How the Catholic Church Built Civilization, and on the resource pages for the Law Unit. An excerpt from the essay is quoted below.
". . . .The Catholic impact on Western law can hardly be overestimated. It was the development of canon law in the Middle Ages that retrieved, transformed, and then re-presented the long-forgotten Justinian code of the ancient Roman Empire to emerging European polities in dire need of good juridical models. Specifically, it was the emergence of ecclesiastical courts after Pope Gregory VII that prompted civil courts to imitate and eventually supersede them.
"This imitation can be seen in several different ways, beginning with the very idea of the rule of law. Although this principle may be found in several ancient civilizations, its reintroduction to the West is the result of the medieval Church. Professor Charles Donahue, Jr., speculates that several unique factors contributed to this development, including . .. the Catholic belief in the transcendence of God and in overarching immutable principles that transcend particular legal codes — in other words, in the reassuring rationality of the Divine Logos and the reasonableness of His order.
"We can also see the impact of Catholicism on the Anglo-American common law tradition. As John C. H. Wu writes, "While the Roman law was a deathbed convert to Christianity, the common law was a cradle Christian. Common law presupposed that the same God who governs the "laws of nature" (such as gravity and generation) also governs the "natural law," those universally binding moral precepts that are knowable to all men and women because they are written on the tablets of their heart (Romans 2:15). . . . Indeed, in 1829 Joseph Story could write, "There never has been a period in which the Common Law did not recognize Christianity as lying at its foundations."
"Another byproduct of the legal system to emerge from the crucible of medieval Catholic tradition is the concept of equity, the fair settlement of grievances and injustices often beyond the purview of statutory law. It was the ecclesiastical courts and religious orders of the Church that introduced the idea of equity to the English realm and hence to Anglo-American law. By combining aspects of Greco-Roman law, Celtic-Saxon custom, and Judeo-Christian principles in their efforts to be fair when adjudicating canon law cases, the Church and its courts became the locus for equitable justice in medieval England. . . . .
"The notions of intent and liability are also derived from a Catholic sensibility. As canon lawyers in the twelfth century began to reflect on the nature of innocence and guilt, they were slowly able to replace the Germanic preference for ordeals by fire and water as a means of deciding culpability with trials that were governed by more rational principles. One of the fruits of this was the development of the concepts of intent and liability in judiciary evaluations, concepts that are still with us today. . . .
"But the Catholic faith has done more than just lend some of its features to civil law; it has also broken new ground. It was Catholic conscience confronting the evils of New World colonialism that led vicariously to the development of the modern system of international law. . . . .
". . . .Father Francisco de Vitoria (1486–1546), was a Thomist scholar whose explication of just war theory essentially rejects a European "right" to conquer the inhabitants of the New World in the name of civilization or Christianity. The alleged abuse or neglect of life and property by Native Americans, Vitoria taught, was not in itself sufficient grounds for waging war on them or subjugating them. . . . Such teachings, which were inspired by the Catholic natural law tradition and the Catholic understanding of the unity of the human race, contributed much to the emergence of international law."