As monumental as the election of a new pope has always been and always will be for Catholics around the world, other (very) occasional events in the life of the church often have far more wide-reaching and long-lasting effects. The pontiff is, of course, the supreme figure in the church’s hierarchy — but more than 260 men have held the title over the past two millennium, and only a relative handful of those figures have presided over historic changes in the ways Catholics worship and relate to both their local parishes and congregations and to the Holy See in Rome.

By contrast, for example, the Second Vatican Council (known colloquially as Vatican II) held at St. Peter’s between 1962 and 1965 was more significant — in both its message and its spiritual and cultural ramifications — than any single act or pronouncement by any pope of the past few centuries. The twenty-first Ecumenical Council of the Catholic Church, and only the second ever held at Saint Peter’s Basilica, Vatican II offered nothing less than an utterly new dialogue between the Roman Catholic Church and the modern world.

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Opened in October 1962, when Pope John XXIII was pontiff, and lasting until December 1965, during the papacy of Pope Paul VI, Vatican II advanced both major and minor reforms to church doctrine and matters of faith, e.g., encouraging Catholics to pray with non-Catholic Christians; a new prominence (and power) for bishops within the church; the sanctioning of languages other than Latin during Mass, and more.

Here, on the eve of Pope John XXIII’s canonization, LIFE.com recalls the historic, landmark Vatican II council with photographs by Hank Walker and Paul Schutzer.