1 MATER ET MAGISTRA ENCYCLICAL OF POPE JOHN XXIII ON CHRISTIANITY AND SOCIAL PROGRESS MAY 15, 1961 To His Venerable Brethren the Patriarchs, Primates, Archbishops, Bishops, and all other Local Ordinaries that are at Peace and in Communion with the Apostolic See, and to the Clergy and Faithful of the entire Catholic World. ..