1878–1899

Leo XIII's encyclicals (Aeterni Patris, Rerum Novarum, Providentissimus Deus) and the condemnation of Catholic modernism

Leo XIII was an "enlightened moderate" between Pius IX and Pius X. In the encyclical Aeterni Patris (1879) he mandated the study of Thomas Aquinas for all theology students. This launched a revival of Thomism. Leo XIII also opened the Vatican archives to scholars.

In Rerum Novarum (1891) he condemned the gulf between the wealth of the few and the poverty of the masses. He affirmed workers' right to a just wage and to form (Catholic) unions. Yet he rejected socialism and defended private property as a God-given right. The encyclical launched Catholic social teaching and the Catholic labor movement.

Providentissimus Deus (1893) cautiously allowed the use of biblical-critical scholarship. Immortale Dei (1885) upheld the ideal of a Catholic state.

During Leo XIII's pontificate, "Catholic modernism" also arose, led by Alfred Loisy and George Tyrrell. It embraced skeptical biblical criticism. The movement was condemned and largely halted under Pius X.