If you didn’t know who he was, Pope John XXIII would have struck you as a most ordinary individual. Although he shunned the limelight as much as possible, this simple man was perhaps the most influential pope to preside over the Catholic Church in the 20th century.

The oldest son in a family of tenant farmers in northern Italy (he was born in 1881), Angelo Roncalli was always proud of his down-to-earth roots. After attending his diocesan seminary, he spent a year serving in the military before being ordained to the priesthood in 1904. He went on to Rome to study Canon Law, working as his bishop’s secretary as well as teaching Church history in seminary.

When World War I erupted in Europe, Father Roncalli was called back to military service as a stretcher bearer and chaplain to the Italian army, an experience which gave him first-hand knowledge of the suffering caused by war.

Following his discharge in 1919, he became the head of the National office of the Society of the Propagation of the Faith. Named an archbishop in 1925, He was appointed as apostolic visitator to Bulgaria, where he came to know the Eastern Catholic Church very well.

During the Second World War, Archbishop Roncalli helped an estimated 24,000 Jews escape Nazi persecution by issuing “transit visas’ from the apostolic delegation. His efforts would earn him the designation of “Righteous Gentile” following the war.

Roncalli was named a Cardinal and patriarch of Venice in 1953 and then, in 1958, just one month shy of his 78th birthday, he was elected as the 261st pope in the Catholic Church, the oldest in nearly 200 years. He took the name “John” after his father.

Although Pope John XXIII was known for two famous encyclicals, “Mater et Magistra” (Mother and Teacher), which emphasized the obligations of nations and individuals to bring about social justice, and “Pacem in Terris” (Peace on Earth) his second social encyclical released at the height of the Cold War, he will be forever associated with the Second Vatican Council. With its first session opening in October 1962, Vatican II set in motion major reforms in the Church, its structure, liturgy, and relations with other Christians and religions. Its central document, “Lumen Gentium,” declared that the Church “was not to be presented as a closed society or institution, but as the people of God who are sent out by Christ to be the ‘light to the nations.’” Another document, “Gaudium et Spes,” emphasized the need to bring the “joy and hope” of Christ to all persons.

Although Pope John XXIII died of stomach cancer only eight months later (June 3, 1963) the council, under the leadership of the next pope, Paul VI, would continue the work of renewal envisioned by Pope John. Beatified by Pope John Paul II in September 2000, he was canonized in 2014 by Pope Francis.

His feast is October 11, the same date as the opening of the Second Vatican Council.