Leo XIII(1878-1903) was certainly one of the most important Popes in modern times, not only for the length of his pontificate, second only to Blessed Pius IX’s, but above all for the extent and richness of his Magisterium. His teaching includes encyclicals of fundamental importance, such as Aeterni Patris (1879) on the restoration of Thomist philosophy, Arcanum (1880) on the indissolubility of marriage, Humanum genus (1884) against Masonry, L’Immortale Dei (1885) on the Christian constitution of the States and Rerum Novarum (1891) on the question of work and social life.
