Pope John XXIII issued eight Papal Encyclicals during his five-year reign as Pope of the Roman Catholic Church, from his election on October 28, 1958 until his death on June 3, 1963. Two of his encylicals, Mater et magistra and Pacem in terris, are especially important. A Papal Encyclical is a letter sent by the Pope which is addressed to Roman Catholic bishops in a particular area or the whole world. Encyclicals may condemn errors, point out threats to faith and morals, exhort faithful practices, or provide remedies for present and future dangers to the church. The authority of the encyclical varies depending on the circumstances and is not necessarily ex cathedra. The title of a Papal Encyclical is usually taken from its first few words.
Pope John XXIII's first encyclical, Ad Petri cathedram, was issued eight months into his pontificate and was neither an important social document nor doctrinal exposition. Instead it looked at truth, unity and peace with distinctive familiarity and concern. The second, Sacerdotii nostri primordia, commemorated the 100th anniversary of the death of St. John Vianney, while Grata recordatio considered the use of the Rosary. Princeps pastorum, his fourth encyclical, used 1 Peter 5:4 as its biblical text and celebrated Roman Catholic missions.
Mater et magistra, the fifth encyclical, carried forward ideas from Leo XIII's Rerum novarum (1891), which had been issued 70 years before, and Pius XI's Quadragesimo anno (1931). It considers social ethics with its most important point being the application of natural law to the international community. It is one of the longest encyclicals, at more than 25,000 words. The sixth encyclical, Aeterna Dei sapientia, commemorated the death of Pope Leo I and called for unity within Christendom from external movements such as Communism and secularism. The penultimate encyclical, Paenitentiam agere, considered penance and the then-upcoming Second Vatican Council. Pope John XXIII's final encyclical, Pacem in terris, was written two months before his death. It is long—over 15,000 words—and is the first in history to have been addressed to "all men of good will," rather than only the bishops and laity of the Roman Catholic Church. It was hailed as "one of the most profound and significant documents of our age."