John XXIII convened the Second Vatican Council in 1962 but died in June 1963 before its work was complete, leaving his successor to decide whether to continue, redirect, or quietly close a council whose scope and direction were still being contested even among the bishops attending it. Paul VI, elected weeks later, chose to continue it without significant change of course, presiding over its remaining three sessions through 1965 and signing every one of its sixteen final documents, on subjects ranging from the liturgy and the nature of the church to religious freedom and relations with non-Christian religions.
Implementing those documents fell to Paul VI alone, since the council itself had no ongoing authority once it closed. He authorized the wholesale revision of the Roman Mass into the form promulgated in 1969 and generally called the Novus Ordo, permitting the vernacular language of each country in place of Latin as the ordinary language of the liturgy, a change more visible to ordinary Catholics in daily practice than almost anything else the council produced, and one that provoked lasting resistance among Catholics attached to the older Latin rite.
Paul VI also broke sharply with centuries of papal custom by traveling abroad, becoming the first pope in the modern era to leave Italy repeatedly, visiting the Holy Land in 1964, addressing the United Nations in New York in 1965, and eventually reaching six continents, a pattern of international travel his successors, especially John Paul II, would expand into a defining feature of the modern papacy.