The Second Vatican Council (1962–65) was the most important event in the history of the Catholic Church since the Council of Trent (1545–63). Vatican II reconceptualized the understanding of the Church and had a significant transformative effect on internal Catholic life and thought and on the Church’s relationship with other Christians, other world religions, and the modern secular world.

At the conclusion of the Week of Prayer for Church Unity (25 January 1959), ninety days after he was elected, Pope John XXIII (1881–1963; pope, 1958–63) made the surprising announcement that he intended to convoke a “general Council for the Universal Church.” At the time of the announcement, the pope had not established an agenda for the new council; nor was it entirely clear what he had in mind. He wanted the universal Church, however, to come to grips “with the spiritual needs of the present time” and hoped that such a council, as had councils in the Church’s previous history, might strengthen “religious unity” and kindle a “more intense Christian fervor.” The internal renewal of Catholic life and the fostering of Christian unity seem to have been foremost in the pope’s mind when he called the council. At the time of this announcement, it was not clear to the cardinals of the Church, to whom the message was communicated, what specifically was intended for the new council, and some of them were not happy with the news.