In the 25 years of his pontificate, there was seldom an area he touched that he didn't leave better off. First, we must note the length of his pontificate. I truly believe this was proof of God's sense of humor. After the 35 year reign of his predecessor Blessed Pius IX, the Church was hoping for a "transition" Pope. There were many who didn't like Pius' very confrontational approach, and were looking to institute a more "progressive" Pope. Rather than dying quickly, Leo XIII lived to the age of 93, and essentially confirmed in place the magesterium of Pius IX. What I hope follows is a very general tracing of the aspects of Church life he touched and left forever changed.
Being a convert who loves the Scriptures, the Church as a whole is indebted to Leo. The serious discipline of Biblical studies in the Catholic Church today traces a lot of their roots back to Leo XIII and his encyclical Providentimus Deus. He boldly stated things that even many Catholics (to say nothing of Protestants!) are surprised by:
For those whose duty it is to handle Catholic doctrine before the learned or the unlearned will nowhere find more ample matter or more abundant exhortation, whether on the subject of God, the supreme Good and the all-perfect Being, or of the works which display His Glory and His love. Nowhere is there anything more full or more express on the subject of the Savior of the world than is to be found in the whole range of the Bible. As St. Jerome says, "To be ignorant of the Scripture is not to know Christ." In its pages His Image stands out, living and breathing; diffusing everywhere around consolation in trouble, encouragement to virtue and attraction to the love of God. And as to the Church, her institutions, her nature, her office, and her gifts, we find in Holy Scripture so many references and so many ready and convincing arguments, that as St. Jerome again most truly says: "A man who is well grounded in the testimonies of the Scripture is the bulwark of the Church.'' And if we come to morality and discipline, an apostolic man finds in the sacred writings abundant and excellent assistance; most holy precepts, gentle and strong exhortation, splendid examples of every virtue, and finally the promise of eternal reward and the threat of eternal punishment, uttered in terms of solemn import, in God's name and in God's own words.It was often joked that seminarian had the Bible, and he had the Summa Theologica, the great work of the Angelic Doctor St. Thomas Aquinas. In Leo XIII the Angelic Doctor found his most forceful patron. In Aeterni Patris, a rigorous study of Aquinas became mandatory in not only seminaries, but Catholic lay universities as well. In the years since, we have seen rapid development of Thomism, including some Thomists who gave us new ways of understanding philosophy such as Dietrich Von Hildreband, and a very young Karol Wojtyła, who would eventually become John Paul II. These two were giants of the "personalist" school of philosophy, and were deeply influenced by Aquinas.
As well as being a pope of great learning, he was also a pope of exemplary piety. Being the first pope of the 20th century, he prayed a very public novena for an outpouring of the Spirit (sometimes even referred to as a "New Pentecost") as the new century approached. I believe the Holy Spirit heard his prayer. The following 60 years could legitimately be viewed a "golden age" of Catholicism, from the 4 outstanding popes after him (including St. Pius X), the explosion of Catholic scholarship, and prolific writers like G.K Chesteron, J.R.R Tolkien and Archbishop Fulton Sheen, who took the Gospel to large audiences.
He wrote at length promoting devotion to Saint Joseph. The increase in devotion to the sainted custodian of Our Lord led 60 years later for his name to be inserted into the canon, and generally being viewed as the patron of all workers everywhere. St. Michael the Archangel also received special devotion in his pontificate, creating the very prayer to St. Michael the Archangel, which, somewhat shortened, many Catholics pray every day.
All of these popular devotions however pale in comparison to the amount of time he spent promoting the devotion rightly due to the Blessed Virgin Mary. Many times he has truly been called the "rosary Pope." He wrote eleven (count it, eleven!) encyclicals on the Rosary. He clearly spelled out our understanding of Mary as co-redemptrix and mediatrix, never taking away or adding anything to Christ's sacrifice in terms of merit, but within the context of her being a model as the example of perfect submission to the Divine Redeemer. The very Marian nature of later popes was heavily influenced by this Pope.
In terms of ecumenism and relationship with the world outside the Church, again great strides were made, but never at the expense of Catholic fidelity. Diplomatic relations were restored and strengthened almost universally during his pontificate. He defended the rights of Eastern Catholics from "Latinization", vieweing their traditions as absolutely essential to the universal Church. Whenever relationships with the Eastern Orthodox are discussed, the impact of Leo XIII is always mentioned.
In matters of Europe, he demonstrated a strong interest in England's Catholic population. He raised John Henry Newman to the rank of Cardinal and canonized the 50 "English Martyrs." One could say he also in a sense delivered what would come to be the "killing blow" against Anglicanism, ruling their priestly ordinations invalid in Apostolicae Curae (a once proud Christian tradition since that time has turned itself into a shade of its former self, now known more for compromising Christianity than anything else.) He was also one of the first Popes to take a keen interest in matters across the Atlantic in the United States of America. While giving great praise towards the (at the time) young republic, he also warned American Catholics to remain on firm ground in their thinking (condemning the heresy of "Americanism", one of the earlier manifestations of Christian pluralism).
All of these things would already make a very successful pontificate. For probably one of the most intelligent popes (worldwide he was known as a true master of Latin and an outstanding writer), he was just getting started. Leo XIII is mostly known for his "social magesterium." Almost everything about Catholic social teaching underwent a massive development under Leo XIII. It had been centuries since a Pope engaged the modern world with the tenacity he displayed. He began a serious defense and study of the sacrament of marriage, a study which needs renewed appreciation today. He defended and developed the understanding of marriage as the foundation for all society and its understanding even of liberty and equality came from this institution. He ardently defended the primacy of the parent in the role of educating their children. He defended the true understanding of liberty, and pointed out the essential Christian character of Western Civilization.
He is best remembered however for his work on socialism. One could say Leo was the first true modern prophet within the Church condemning socialism. In all of his political encyclicals, he attacked socialism. All of these culminated in the most influential of all modern encyclicals, Rerum Novarum. In this great work (which 4 popes have commemorated with special anniversary encyclicals), he gives the Catholic Churches definitive response to Marxism and the Communist Manifesto. He rejects socialism as utterly incompatible with Christian thought, and predicts (with almost frightening accuracy) what will happen to countries that embrace socialism.
He then stunned the world by also going after capitalism. While praising the market, he also warns that it is not the answer for everything, and that a rugged individualism is every bit as dangerous as socialism. He defends the rights of workers and labor unions, provided they are done with respect to Catholic conscience. Catholic economic thought entered another golden age as a result of this encyclical. Even today, Catholic scholars invoke the memory of this great Pope in trying to understand economics from a Catholic perspective.
The truth is, I could go on and on for ages extolling the benefits of studying Leo XIII. I believe I've given a strong outline for why I think he is such a solid pope. Yet, even with all of these accomplishments, even with so many turning to him for guidance throughout the years, I would say he is a "forgotten" pope in the minds of the average Catholic. They know Pius XII, they know Paul VI, they might be familiar with Pius IX and St. Pius X, they are certainly familiar with John Paul II. Many of these individuals owe a significant amount to the work of Leo XIII. As we celebrate the 200th year of his birth, let our devotion and understanding of his work only increase. While not a canonized saint, I believe there is little doubt where he resides, and that he to this day intercedes with us before the source of wisdom he so eloquently put forth.