Humanae Vitae: Encyclical Letter of Supreme Pontiff Paul VI

The most serious duty of transmitting human life, for which married persons are the free and responsible collaborators of God the Creator, has always been a source of great joys to them, even if sometimes accompanied by not a few difficulties and distress. At all times the fulfillment of this duty has posed grave problems to the conscience of married persons, but, with the recent evolution of society, changes have taken place that give rise to new questions which the Church could not ignore, having to do with a matter which so closely touches upon the life and happiness of men.