Pope John XXIII issued Pacem in terris, the first papal encyclical addressed to "all men of good will" rather than only to Catholics.

Pope John XXIII was head of the Catholic Church and sovereign of the Vatican City State from 28 October 1958 until his death on 3 June 1963. He is the most recent pope to take the pontifical name "John".

Pacem in terris is a papal encyclical issued by Pope John XXIII on 11 April 1963, on the rights and obligations of people and their states, as well as proper interstate relations. It emphasizes human dignity and human equality in endorsing women's rights, nuclear nonproliferation and the United Nations.

An encyclical was originally a circular letter sent to all the churches of a particular area in the ancient Roman Church. At that time, the word could be used for a letter sent out by any bishop. The word comes from the Late Latin encyclios. The term is now primarily associated with papal encyclicals.