Born 2 March 1810, at Carpineto. Gioacchino Vincenzo Raffaele Luigi was the sixth of the seven sons of Count Lodovico Pecci and his wife Anna Prosperi-Buzi. There was some doubt as to the nobility of the Pecci family, and when the young Gioacchino sought admission to the Accademia dei Nobili in Rome he met with a certain opposition, whereupon he wrote the history of his family, showing that the Pecci of Carpineto were a branch of the Pecci of Siena, obliged to emigrate to the Papal States in the first half of the sixteenth century, under Clement VII, because they had sided with the Medici.
His zeal and ability convinced Cardinal Sala that Pecci was fitted for larger responsibilities, and he urged him to enter the priesthood, hinting in addition that before long he might be promoted to a post where the priesthood would be necessary. Yielding to these solicitations, he was ordained priest 31 Dec., 1837, by Cardinal Odeschalchi, Vicar of Rome, in the chapel of St. Stanislaus on the Quirinal.
In August, 1877, on the death of Cardinal de Angelis, Pius IX appointed him camerlengo, so that he was obliged to reside in Rome. Pope Pius died 7 February, 1878, and during his closing years the Liberal press had often insinuated that the Italian Government should take a hand in the conclave and occupy the Vatican. However the Russo-Turkish War and the sudden death of Victor Emmanuel II (9 January, 1878) distracted the attention of the Government, the conclave proceeded as usual, and after the three scrutinies Cardinal Pecci was elected by forty-four votes out of sixty-one
Under Leo the Catholic Faith made great progress; during his pontificate two hundred and forty-eight episcopal or archiepiscopal sees were created, and forty-eight vicariates or prefectures Apostolic. Catholics of Oriental rites were objects of special attention; he had the good fortune to see the end of the schism which arose in 1870 between the Uniat Armenians and ended in 1879 by the conversion of Mgr. Kupelian and other schismatical bishops. He founded a college at Rome for Armenian ecclesiastical students (1884), and by dividing the college of S. Atanasio he was able to give the Ruthenians a college of their own; already in 1882 he had reformed the Ruthenian Order of St. Basil; for the Chaldeans he founded at Mossul a seminary of which the Dominicans have charge. In a memorable encyclical of 1897, he appealed to all the schismatics of the East, inviting them to return to the Universal Church, and laying down rules for governing the relations between the various rites in countries of mixed rites. Even among the Copts his efforts at reunion made headway. The ecclesiastical sciences found a generous patron in Pope Leo. His Encyclical "Æterni Patris" (1880) recommended the study of Scholastic philosophy, especially that of St. Thomas Aquinas, but he did not advise a servile study. In Rome he established the Apollinare College, a higher institute for the Latin, Greek, and Italian classics. At his suggestion a Bohemian college was founded at Rome. At Anagni he founded and entrusted to the Jesuits a college for all the dioceses of the Roman Campagna, on which are modelled the provincial or "regional" seminaries desired by Pius X. Historical scholars are indebted to him for the opening of the Vatican Archives (1883), on which occasion he published a splendid encyclical on the importance of historical studies, in which he declares that the Church has nothing to fear from historical truth. It was under his pontificate that the foundation for modern Biblical scholarship was formed. The German scholar, Julius Wellhausen, was the first to popularize the idea that the Bible, particularly the Pentateuch, may not have been written by one person, or Moses. For the administration of the Vatican Archives and Library he called on eminent scholars (Hergenröther, Denifle, Ehrle; repeatedly he tried to obtain Janssen, but the latter declined, as he was eager to finish his "History of the German People"). For the convenience of students of the archives and the library he established a consulting library. The Vatican Observatory is also one of the glories of Pope Leo XIII. To excite Catholic students to rival non-Catholics in the study of the Scriptures, and at the same time to guide their studies, he published the "Providentissimus Deus" (1893), which won the admiration even of Protestants, and in 1902 he appointed a Biblical Commission. Also, to guard against the dangers of the new style of apologetics founded on Kantism and now known as Modernism, he warned in 1899 the French clergy (Encycl. "Au Milieu"), and before that, in a Brief addressed to Cardinal Gibbons, he pointed out the dangers of certain doctrines to which had been given the name of "Americanism" (22 Jan., 1899). In the Brief "Apostolicæ Curæ" (1896) he definitively decided against the validity of Anglican Orders. In several other memorable encyclicals, he treated of the most serious questions affecting modern society. They are models of classical style, clearness of statement, and convincing logic. The most important are: "Arcanum divinæ sapientiæ" (1880) on Christian marriage; "Diuturnum illud" (1881), and "Immortale Dei" (1885) on Christianity as the foundation of political life; "Sapientiæ christianæ" (1890) on the duties of a Christian citizen; "Libertas" (1888) on the real meaning of liberty; "Humanum genus" (1884) against Freemasonry (he also issued other documents bearing on this subject).
From his youth, Pope Leo XIII had a strong devotion to Mary. Through the discovery of the Marian writings of St. Louis de Montfort in 1846, and the subsequent investigations into these writings as part of de Montfort’s beatification cause, Pope Leo XIII was deeply influenced by de Montfort’s Marian thought. He was so enamored with True Devotion to the Blessed Virgin that he granted an indulgence to anyone who consecrated themselves to Mary using St. Louis de Montfort’s method. Another source of Marian inspiration for Pope Leo XIII was the work of Blessed Bartolo Longo in Pompeii.
Pope Leo XIII is the greatest champion of the rosary to ever hold the office of the Vicar of Christ. During his pontificate, he wrote 11 encyclicals on the rosary, promulgated numerous apostolic letters on the rosary, and gave countless messages on the rosary to various dioceses and religious institutes. His rosary encyclicals contain a summary of all the statements previous popes had made about St. Dominic’s role as the father of the rosary and the founder of the Confraternity of the Rosary. In almost every rosary encyclical that he wrote, he affirmed that St. Dominic was the founder of the rosary. He expressly taught that Our Lady herself entrusted the rosary to St. Dominic and compared St. Dominic’s Confraternity to an army of prayer and a spiritual battalion capable of winning souls for Christ.
In his Encyclical Supremi Apostolatus Officio, on devotion of the Rosary - September 1, 1883 , he writes;
“There is none among you, venerable brethren, who will not remember how great trouble and grief God's Holy Church suffered from the Albigensian heretics, who sprung from the sect of the later Manicheans, and who filled the South of France and other portions of the Latin world with their pernicious errors, and carrying everywhere the terror of their arms, strove far and wide to rule by massacre and ruin. Our merciful God, as you know, raised up against these most dire enemies a most holy man, the illustrious parent and founder of the Dominican Order.
Great in the integrity of his doctrine, in his example of virtue, and by his apostolic labors, he proceeded undauntedly to attack the enemies of the Catholic Church, not by force of arms; but trusting wholly to that devotion which he was the first to institute under the name of the Holy Rosary, which was disseminated through the length and breadth of the earth by him and his pupils. Guided, in fact, by divine inspiration and grace, he foresaw that this devotion, like a most powerful warlike weapon, would be the means of putting the enemy to flight, and of confounding their audacity and mad impiety. Such was indeed its result. Thanks to this new method of prayer - when adopted and properly carried out as instituted by the Holy Father St. Dominic - piety, faith, and union began to return, and the projects and devices of the heretics to fall to pieces. Many wanderers also returned to the way of salvation, and the wrath of the impious was restrained by the arms of those Catholics who had determined to repel their violence.”
He died in the middle of the “Modernist” crisis in the Roman Church on July 20, 1903, but his legacy lives on in the scientific study of the Bible.