Pope Leo XIII (born Vincenzo Gioacchino Raffaele Luigi Pecci; 2 March 1810 – 20 July 1903) was the head of the Catholic Church from 20 February 1878 to his death in July 1903. He was a great diplomat who managed to improve relations between the Catholic Church and the modern world. He also performed a number of consecrations, at times entering new theological territory.
Pope Leo XIII used all his authority for a revival of Thomism, the theology of Thomas Aquinas. On 4 August 1879, he promulgated the encyclical Aeterni Patris ("Eternal Father") which provided a charter for the revival of Thomism as the official philosophical and theological system. He mandated all Catholic Universities to teach Thomism and created a papal academy for the training of Thomists professors and re-edited scholarly editions of Thomas Aquinas.
In 1886, Pope Leo XIII decreed that a prayer to St. Michael be said at the end of "low" Mass (not "high", or sung Masses) throughout the universal Church, along with the Salve Regina (Hail, Holy Queen). In 1885 he published Immortale Dei, dispelling immediately any notion that the Church is opposed to civil government. He recognized two powers — ecclesiastical and civil — which exercise control over human affairs and located the Church’s position outside of capitalism versus socialism dialectic.
Pope Leo XIII's pontificate moved the Catholic Church back to mainstream European life and he is credited with restoring much prestige and authority to papacy by his theological teachings