An encyclical was originally a circular letter sent to all the churches of a particular area in the ancient Catholic Church. At that time, the word could be used for a letter sent out by any bishop. The word comes from Latin encyclicus (from the Greek ἐν κύκλῳ en kykloi) meaning "general" or "encircling", which is also the origin of the word "encyclopedia".

The term has been used by Catholics, Anglicans and the Eastern Orthodox.

For the modern Catholic Church a Papal encyclical, in the strictest sense, is a letter, usually treating some aspect of Catholic doctrine, sent by the Pope and addressed either to the Catholic bishops of a particular area or, more normally, to the bishops of the world; however, the form of the address can vary widely, and often designates a wider audience. Papal encyclicals usually take the form of a Papal brief due to their more personal nature as opposed to the formal Papal bull. Papal encyclicals are so famous that the term encyclical is used almost exclusively for those sent out by the Pope. The title of the encyclical is usually taken from its first few words (its incipit).

Blessed Pope John XXIII (Latin: Ioannes PP. XXIII; Italian: Giovanni XXIII), born Angelo Giuseppe Roncalli (25 November 1881 – 3 June 1963), headed the Catholic Church and ruled Vatican City from 1958 until his death.

Pope John was elected on 28 October 1958. He called the Second Vatican Council (1962–1965) but did not live to see it to completion. He died in 1963, only four-and-a-half years after his election, and two months after the completion of his final encyclical, Pacem in Terris. He was beatified, along with Pope Pius IX, on 3 September 2000.

Angelo Giuseppe Roncalli was born in Sotto il Monte, a small country village in the Bergamo province of the Lombardy region of Italy. He was the first-born son of Giovanni Battista Roncalli (1854–1935) and his wife Marianna Giulia Mazzolla (1854–1939), and fourth in a family of 13, including: Angelo Giuseppe, Alfredo (1889–??), Maria Caterina (1877–1883), Teresa (1879–1954), Ancilla (1880–1953), Domenico Giuseppe (22 February 1888 – 14 March 1888), Francesco Zaverio (1883–1976), Maria Elisa (1884–1955), Assunta Casilda (1886–??), Giovanni Francesco (1891–1956), Enrica (1893–1918), Giuseppe Luigi (1894–??) and Luigi (1896–1898). His family worked as sharecroppers as did most of the people of Sotto il Monte – a striking contrast to that of his predecessor, Eugenio Pacelli (Pope Pius XII), who came from an ancient aristocratic family, long connected to the Papacy. However, he was still a descendant of an Italian noble family, from a secondary and impoverished branch.

Paul VI (Latin: Paulus PP. VI; Italian: Paolo VI), born Giovanni Battista Enrico Antonio Maria Montini (26 September 1897 – 6 August 1978), reigned as Pope of the Catholic Church from 21 June 1963 until his death on 6 August 1978. Succeeding Pope John XXIII, who had convened the Second Vatican Council, he decided to continue it. He fostered improved ecumenical relations with Orthodox and Protestants, which resulted in many historic meetings and agreements.

Montini served in the Vatican's Secretariat of State from 1922 to 1954. While in the Secretariat of State, Montini and Domenico Tardini were considered as the closest and most influential co-workers of Pope Pius XII, who named him in 1954 Archbishop of Milan, the largest Italian diocese, yet denying him the Cardinal designation that traditionally accompanies being Archbishop of Milan, a function which made him automatically Secretary of the Italian Bishops Conference. John XXIII elevated him to the College of Cardinals in 1958, and after the death of John XXIII, Montini was considered one of his most likely successors.