Elected pope in June, 1963, Paul immediately demonstrated his intention of continuing the reforms of his predecessor, John XXIII. He reconvened the Second Vatican Council (see Vatican Council, Second) and supervised the carrying out of many of its reforms, such as the vernacularization and reform of the liturgy. With the aim of continuing the work of the council after it ended, he instituted an international synod of bishops, and bishops were instructed to set up councils of priests in their own dioceses. In addition, considerable powers of dispensation were devolved from the Roman Curia onto the bishops, the rules on fasting and abstinence were relaxed, and some of the restrictions on intermarriage were lifted. A commission on canon law revision was also established.

In 1964, Paul VI made a pilgrimage to the Holy Land; he was the first pope in over 150 years to leave Italy. That historic journey was followed by trips to India (1964), the United States (1965), where he addressed the United Nations, and other parts of the world, including Africa (1969) and Southeast Asia (1970). Relations between the Vatican and the Communist world were improved; Communist leaders visited the Vatican for the first time, and in 1971 Cardinal Mindszenty, whose presence in the U.S. embassy in Budapest had long bedeviled church-state relations in Hungary, was finally persuaded to go to Rome.

The broader international outlook of the Vatican under Paul VI was matched by a new ecumenism. The pope met with the leaders of other churches and addressed (1969) the World Council of Churches, and limited doctrinal agreements were reached with the Anglicans and Lutherans. Such accords, however, did not represent any modification of the papal claim to spiritual leadership of the whole Christian Church, nor of the doctrine of papal infallibility. In fact, Pope Paul issued frequent reassertions of papal primacy in the face of growing dissent within the Roman Catholic Church itself.

In 1968, in the encyclical Humanae Vitae, Paul reaffirmed the church's long-standing ban on contraception. The encyclical, a disappointment to many liberals within the church, raised a storm of protest, and many national hierarchies openly modified the statement. In the ferment that ensued, liberals also raised questions about priestly celibacy, divorce, and the role of women in the church—all issues on which Paul upheld the traditional position of the church. The dispute developed into a real contest of strength between the Vatican and the Dutch hierarchy in particular, which in 1970 endorsed the marriage of priests and the admission of women into the priesthood. The synod of bishops in 1971 supported the pope's stand on priestly celibacy, but a sizable minority were opposed. At the synod of bishops of 1974, assembled to discuss evangelization in the modern world, Pope Paul disapproved the bishops' proposal for greater autonomy for the local churches.