"... each and every marriage act (quilibet matrimonii usus) must remain open to the transmission of life."
And now, beloved sons, you who are priests, you who in virtue of your sacred office act as counselors and spiritual leaders both of individual men and women and of families—We turn to you filled with great confidence. For it is your principal duty—We are speaking especially to you who teach moral theology—to spell out clearly and completely the Church's teaching on marriage. In the performance of your ministry you must be the first to give an example of that sincere obedience, inward as well as outward, which is due to the magisterium of the Church. For, as you know, the pastors of the Church enjoy a special light of the Holy Spirit in teaching the truth. And this, rather than the arguments they put forward, is why you are bound to such obedience. Nor will it escape you that if men's peace of soul and the unity of the Christian people are to be preserved, then it is of the utmost importance that in moral as well as in dogmatic theology all should obey the magisterium of the Church and should speak as with one voice. Therefore We make Our own the anxious words of the great Apostle Paul and with all Our heart We renew Our appeal to you: "I appeal to you, brethren, by the name of our Lord Jesus Christ, that all of you agree and that there be no dissensions among you, but that you be united in the same mind and the same judgment."29. Now it is an outstanding manifestation of charity toward souls to omit nothing from the saving doctrine of Christ; but this must always be joined with tolerance and charity, as Christ Himself showed in His conversations and dealings with men. For when He came, not to judge, but to save the world, was He not bitterly severe toward sin, but patient and abounding in mercy toward sinners?
Husbands and wives, therefore, when deeply distressed by reason of the difficulties of their life, must find stamped in the heart and voice of their priest the likeness of the voice and the love of our Redeemer.
So speak with full confidence, beloved sons, convinced that while the Holy Spirit of God is present to the magisterium proclaiming sound doctrine, He also illumines from within the hearts of the faithful and invites their assent. Teach married couples the necessary way of prayer and prepare them to approach more often with great faith the Sacraments of the Eucharist and of Penance. Let them never lose heart because of their weakness.
The "radioactivity" metaphor is poignant. In the 40 years since it came out, some have treated the encyclical like it really was "radioactive", and avoided it altogether. Others have been brave enough to draw near to it, knowing that, if handled properly, its power could be harnessed. But for most people, Humanae Vitae, like nuclear power, is something they simply don't know enough about.
That can change. Today, click on this link and read Humanae Vitae for yourself. Remember the time it was written, 1968, and what the world was like at that time.
Read it and it may give you a new opinion about Pope Paul's courageousness and foresight. Read paragraph 17 and decide for yourself whether, in 2008, Papa Montini could be considered a prophet because of what he wrote 40 years ago about what the consequences of a "contraceptive culture" would be.