Pope Leo XIII
Pope Leo XIII was Bishop of Rome from 1878 to 1903. In the course of his twenty-five years as pope, he issued a number of encyclicals on such topics as civil government, socialism, Christian citizenship, the evils affecting modern society, slavery, and Christian democracy. He is most remembered, however, for his 1891 encyclical Rerum Novarum, On the Condition of Labor. This encyclical has become universally recognized as the inaugural document of the tradition of Catholic social teaching and the benchmark against which all subsequent documents have been evaluated.
Joseph N. Moody. Church and Society (1953) 49. "The pontificate of Leo XIII marked the beginning of an official effort to restate the traditional social teaching of the Catholic Church in the context of modern industrialism. The social encyclicals did not initiate the movement. In large part they were the consequence of pioneering by social-minded lay and clerical Catholics in many of the countries affected by the new conditions. But with Leo's reign, the movement 'captured' the papacy in the way that the eleventh century reform had 'captured' Leo IX or the reform of the sixteenth century had Paul III. In 1878, the See of Peter was occupied by a man who was familiar with the broad social currents of the age and who was personally sympathetic to the needs of the industrial proleteriat. From this time on, social issues were to receive major papal attention."
Kathryn Lamboley. National Catholic Reporter (1979) 37. "When Leo became pope in 1878, there was little harmony between the people and the papacy. During the funeral procession of his predecessor, Pius IX, a cursing, stone-throwing mob followed the funeral cortege to the body's final resting place at San Lorenzo Outside the Walls and threatened to throw the dead pope's body into the Tiber River.... When (Leo) died on July 20, 1903, flags flew at half mast in major cities around the world. His proposals had gone to the heart of major human concern--justice for the worker."
Rerum Novarum: One Hundred Years of Catholic Social Teaching John Coleman and Gregory Baum
Leo XIII, and the Social Problem Irish Ecclesiastical Record, 1892, pages 55, 385, 769