By the time Leo XIII was elected in 1878, Catholic philosophy had spent more than a century responding defensively to the Enlightenment, German idealism, and the rise of secular science, often in a fragmented and reactive way rather than from a coherent intellectual center. Leo himself had studied under Italian Jesuits committed to a smaller nineteenth century revival of interest in Thomas Aquinas, and as pope he made that revival official policy. His 1879 encyclical Aeterni Patris urged Catholic seminaries and universities across the world to make the study of Thomas Aquinas central to the training of clergy and the church's engagement with modern philosophy and science.
The encyclical did not ask for uncritical repetition of medieval conclusions but for a return to Aquinas's method, a confidence that reason and revelation, properly understood, could not contradict one another, and a systematic body of thought robust enough to engage contemporary science and philosophy on their own terms rather than simply condemning them. The result, generally called neo-Thomism or neo-scholasticism, shaped Catholic seminary education for most of the following century and produced a wave of scholarship, translation, and philosophical work built on Aquinas's texts.
Leo's own intellectual formation made the choice a natural one, but its effect went well beyond personal preference: it gave the nineteenth and twentieth century church a single authoritative philosophical reference point at a moment when it might otherwise have fragmented into competing responses to modern thought, and it directly shaped the theological training of the bishops and theologians who would later staff the Second Vatican Council.