Why did Pope Leo XIII call for renewed study of St. Thomas Aquinas?

In his 1879 encyclical Aeterni Patris, Pope Leo XIII called for a restoration of the philosophy of St. Thomas Aquinas primarily to provide a robust defense of the Catholic Faith against modern intellectual challenges.1

By the mid-19th century, church authorities were concerned that modern idealist and subjectivist philosophies—such as those of Immanuel Kant and G.W.F. Hegel—were undermining Christian doctrine. Pope Leo XIII viewed the realism found in the works of Aquinas and Aristotle as a necessary "bulwark" to counter these trends, affirming the stability of the external world and the importance of sound doctrine in addressing contemporary social and intellectual problems.1

The Pope’s program encouraged an accurate historical study of Aquinas, combined with the integration of reliable modern research. This initiative effectively established Thomism as the official philosophical framework of the Roman Catholic Church, a status solidified when Leo XIII named Aquinas the patron of all Catholic schools in 1880.12