31 Venerable brothers, beloved sons, all men of good will, great indeed is the work of education, of progress and of charity to which We now summon all of you. And this We do relying on the unshakable teaching of the Church, which teaching Peter's successor together with his brothers in the Catholic episcopate faithfully guards and interprets. And We are convinced that this truly great work will bring blessings both on the world and on the Church. For man cannot attain that true happiness for which he yearns with all the strength of his spirit, unless he keeps the laws which the Most High God has engraved in his very nature. These laws must be wisely and lovingly observed. On this great work, on all of you and especially on married couples, We implore from the God of all holiness and pity an abundance of heavenly grace as a pledge of which We gladly bestow Our apostolic blessing. Given at St. Peter's, Rome, on the 25th day of July, the feast of St. James the Apostle, in the year 1968, the sixth of Our pontificate.
Source: Humanae Vitae (Vatican.va)