Columbia subject headings:
Leo XIII, Pope, 1810-1903.
Harvard subject headings:
Church and social problems--Catholic Church--History--19th century.
Kulturkampf.
Leo XIII, Pope, 1810-1903
Modernism (Christian theology)--Catholic Church--History.
Papacy--History
Papacy--History--19th century.
Popes--Biography
Princeton subject headings:
Leo XIII, Pope, 1810-1903.