As so-called periti (expert advisers), about two hundred theologians (generally professors of theology) were called to Rome. Of the 2,908 legitimate delegates, 2,540 participated in the opening. Of the invited non-Catholic Christian churches and communities, seventeen were present through thirty-five representatives. In the end, twenty-eight non-Roman churches, including the Russian Orthodox church, were represented by ninety-three observers. There were eighty-six governments and international bodies represented at the opening.
The council met in four sessions: October 11 to December 8, 1962; September 29 to December 2, 1963; September 14 to November 21, 1964; and September 4 to December 8, 1965. Ten public sessions and 168 general assembly meetings were held.
From the time of its proclamation, the council was intended to have a double goal: reform within the Church and preparation for Christian and world unity. But already in the opening address this goal was expanded and deepened. The council was given the task of proclaiming the entire Christian truth "through a new effort" whereby it was of great consequence to distinguish between the truths and the "way in which they are proclaimed." Aggiornamento (Ital., "coming together") was to demonstrate the credibility of the Church, and the Church's relationship to non-Christians was to be improved.
- New Catholic Encyclopedia, Karl Rahner and Adolf Darlap
John XXIII signs Humanae Salutis, the bull convoking the Council. December 25, 1961.