2 Our recent predecessors did not fail to do their duty in this area. Their noteworthy messages shed the light of the Gospel on contemporary social questions. There was Leo XIII's encyclical Rerum Novarum , (1) Pius XI's encyclical Quadragesimo Anno , (2) Pius XII's radio message to the world, (3) and John XXIII's two encyclicals, Mater et Magistra (4) and Pacem in Terris . (5)
Source: Populorum Progressio (Vatican.va)