Pope Leo XIII (Italian: Leone XIII; born Gioacchino Vincenzo Raffaele Luigi Pecci; 2 March 1810 – 20 July 1903) was head of the Catholic Church from 20 February 1878 until his death in July 1903. He had the fourth-longest reign of any pope, behind those of Peter the Apostle, Pius IX (his immediate predecessor) and John Paul II.

He is well known for his intellectualism and his attempts to define the position of the Catholic Church with regard to modern thinking. In his famous 1891 encyclical Rerum novarum, Pope Leo outlined the rights of workers to a fair wage, safe working conditions, and the formation of trade unions, while affirming the rights to property and free enterprise, opposing both socialism and laissez-faire capitalism. With that encyclical, he became popularly titled as the "Social Pope" and the "Pope of the Workers", also having created the foundations for modern thinking in the social doctrines of the Catholic Church, influencing the thoughts of his successors. He influenced the Mariology of the Catholic Church and promoted both the rosary and the scapular. Upon his election, he immediately sought to revive Thomism, the theological system of Augustine of Hippo and Thomas Aquinas, desiring to refer to it as the official political, theological and philosophical foundation for the Catholic Church. As a result, he sponsored the Editio Leonina in 1879.

Leo XIII is particularly remembered for his belief that pastoral activity in political sociology was also a vital mission of the church as a vehicle of social justice and maintaining the rights and dignities of the human person. Leo XIII issued a record of eleven papal encyclicals on the rosary, earning him the title of the "Rosary Pope". In addition, he approved two new Marian scapulars. He was the first pope to have never held any control over the Papal States, which had been dissolved by 1870, since Stephen II in the 8th century. Similarly, many of his policies were oriented towards mitigating the loss of the Papal States in an attempt to overcome the loss of temporal power, but nonetheless continuing the Roman Question. After his death in 1903, he was buried in the Vatican Grottoes before his remains were later transferred in 1924 to the Basilica of Saint John Lateran.

Early life and education, 1810–1836

Gioacchino Pecci was born in Carpineto Romano, near Rome, the sixth of the seven children of Count Domenico Ludovico Pecci (2 June 1767 – 8 March 1833), Patrician of Siena, Colonel of the French Army under Napoleon, and his wife Anna Francesca Prosperi-Buzzi (1773 – 9 August 1824). His uncle Giuseppe Pecci was a protonotary apostolic and referendary of the Signature of Justice and died in 1806. His brothers included Giuseppe and Giovanni Battista or Giambattista Pecci (26 October 1802 - 28 March 1882), 1st Count Pecci, who married on 8 August 1851 Angela Salina (7 February 1830 - 9 October 1899) and had issue, and sister Anna Maria Pecci, wife of Michelangelo Pecci. Until 1818, he lived at home with his family "in which religion counted as the highest grace on earth, as through her, salvation can be earned for all eternity". Together with Giuseppe, he studied in the Jesuit College in Viterbo until 1824. He enjoyed Latin and was known to have written his own Latin poems at the age of eleven. Leo was a descendant of the Italian leader Cola di Rienzo on his mother's side.

Count and Countess Pecci, parents

His siblings were:

• Carlo (1793–1879)
• Anna Maria (1798–1870)
• Caterina (1800–1867)
• Giovanni Battista (1802–1881)
• Giuseppe (1807–1890)
• Fernando (1813–1830)

In 1824, he and Giuseppe were called to Rome, where their mother was dying. Count Pecci wanted his children near him after the loss of his wife and so they stayed with him in Rome and attended the Jesuit Collegium Romanum.

In 1828, the 18-year-old Vincenzo decided in favour of secular clergy, and Giuseppe entered the Jesuit order. Vincenzo studied at the Academia dei Nobili, mainly diplomacy and law. In 1834, he gave a student presentation, attended by several cardinals, on papal judgments. For his presentation, he received awards for academic excellence and gained the attention of Vatican officials. Cardinal Secretary of State Luigi Lambruschini introduced him to Vatican congregations. During a cholera epidemic in Rome, he assisted Cardinal Giuseppe Antonio Sala in his duties as overseer of all the city hospitals. In 1836, he received his doctorate in theology and doctorates of civil and Canon Law in Rome.

Pope Leo XIII (1810–1903)

  March 2, 1810: Birth
Joachim Vincenzo Raffaele Luigi Pecci is born in Carpineto Romano, in the Papal States (modern-day Italy). He is the youngest of seven children in an Italian aristocratic family.

December 31, 1837: Priestly Ordination
After studying at Jesuit schools and the University of Rome, Joachim Pecci is ordained a priest. He stands out for his intellect and administrative skills, paving the way for influential roles in the Church.

January 27, 1843: Appointed Bishop
Joachim Pecci is appointed Archbishop of Perugia. He works actively to improve the local Church’s administration and education, while also focusing on social issues.

February 20, 1878: Becomes Pope
Following the death of Pius IX, Joachim Pecci is elected pope, taking the name Leo XIII. At 67 years old, despite frail health, he leads the Church for 25 years.

May 15, 1891: Publication of the Encyclical Rerum Novarum
Leo XIII publishes the encyclical Rerum Novarum, considered the foundation of modern Catholic social teaching. It addresses workers’ rights, the challenges of capitalism and socialism, and calls for justice in society.

1896: Restoration of the Vatican Observatory
Leo XIII restores the Vatican Observatory, emphasizing that faith and science are not in conflict. This step helps modernize the Church’s image and fosters dialogue with the scientific community.

1878–1903: Diplomacy and Global Relations
Leo XIII improves relations with European nations, including Russia, Germany, and France, as well as the United States and Brazil. He encourages dialogue with Eastern Orthodox churches to bridge historical divides.

July 20, 1903: Death
Leo XIII dies at the age of 93 in the Apostolic Palace in Rome, the oldest pope in Church history. His pontificate lasts 25 years, leaving a lasting impact on the Church.

Legacy
Leo XIII is regarded as one of the most influential popes of the 19th century. He revives Thomistic theology, promotes devotion to Mary, and opens the Vatican Secret Archives to researchers. His encyclical Rerum Novarum continues to influence Catholic social teaching.