By the mid 1960s, the widespread availability of the contraceptive pill had made the question of artificial birth control urgent for millions of Catholic couples, and Paul VI had appointed a commission of theologians, doctors, and laypeople to study the question, whose majority report was widely understood to lean toward permitting some form of artificial contraception within marriage. Paul VI set the majority recommendation aside and, in the 1968 encyclical Humanae Vitae, reaffirmed the church's traditional teaching that each marital act must remain open to the possibility of life, ruling out artificial contraception while approving natural methods of family planning based on periodic abstinence.
The reaction was immediate and, in parts of the Catholic world, unusually open. Theologians in several countries publicly dissented, some episcopal conferences issued statements softening how the teaching should be applied in pastoral practice, and surveys in the following years showed large numbers of practicing Catholics in Europe and North America not following the encyclical's teaching in their own lives, a gap between official doctrine and lay practice that has persisted for more than fifty years and that later popes have addressed with varying degrees of emphasis without reversing the core teaching.
Paul VI himself is said to have been shaken by the scale and openness of the dissent, and he issued no further encyclical in the remaining decade of his pontificate, a silence some historians read as caution after the reception Humanae Vitae received. He was canonized a saint by Pope Francis in October 2018, fifty years after the encyclical's publication.