The most recent issue of the Catholic Medical Quarterly (UK) is dedicated to the 45th anniversary of Humanae Vitae. Included as an editorial is the last known letter by Padre Pio, written to Pope Paul VI on September 12, 1968:
… I know that your heart is suffering much these days … for the lack of obedience of some, even Catholics, to the high teaching that you, assisted by the Holy Spirit and in the name of God, are giving us. I offer you my prayers and daily sufferings as a small but sincere contribution on the part of the least of your sons in order that God may give you comfort with his Grace to follow the straight and painful way in the defense of eternal truth, which never changes with the passing of the years. Also, in the name of my spiritual children and the Prayer Groups, I thank you for your clear and decisive words that you especially pronounced in the last encyclical "Humanae Vitae"; and I reaffirm my faith, my unconditional obedience to your illuminated directions.
Read the entire letter. Among the other many excellent articles is an essay, "The infallible teaching of Humanae Vitae", by Fr Thomas Crean O.P. Fr. Crean argues that the encyclical meets the criteria for infallibility articulated by Vatican I:
To return to Humanae Vitae. Does its teaching on contraception pill fulfil the four conditions that Vatican I laid down for a papal teaching to be infallible? We can consider them in turn.
1.The Pope must be exercising his office of ‘shepherd and teacher of all Christians’, not, for example, giving a Sunday sermon or a lecture to theological students. Humanae Vitae meets this condition and even goes beyond it. Pope Paul addresses his encyclical to “the Venerable Patriarchs, Archbishops, Bishops and other local Ordinaries in peace and communion with the Apostolic See, to priests, the faithful and all men of good will”. Pope Paul was acting as universal shepherd, just as Christ had said to St Peter, without any limitation, ‘Feed my sheep’ (Jn. 21:17).
2. He must be ‘defining a doctrine with his supreme apostolic authority’. He must not, in other words, be proposing a doctrine as merely probably true or as simply safe to follow. As the Relator explained at Vatican I, to define a doctrine the Pope must, using the authority that he alone of mortal men possesses, “directly and conclusively pronounce his judgement... in such a way that each one of the faithful can be certain” of his meaning. In Humanae Vitae¸ Pope Paul explicitly invoked his apostolic authority. “After mature reflection and assiduous prayers”, he wrote, “We now intend, by virtue of the mandate entrusted to us by Christ, to give our reply to these grave questions” (para. 6). He also gave his reply ‘directly and conclusively’: all forms of abortion, sterilization and contraception are to be ‘wholly rejected’ as ‘intrinsically wrong’, and whoever disagrees is ‘entirely mistaken’ (para. 14). Surely, no one who reads Humanæ Vitæ need be in any doubt as to ‘the mind of the Roman pontiff’.
3. The Pope must be speaking about a matter of faith or morals, and not, for example, giving his opinion about literature or secular history. This condition is obviously fulfilled by Humanae Vitae, which gave a teaching on the morality of contraception.
4. He must intend that his teaching be accepted as true by the whole Church. This condition is barely separable from the first two, since if the Pope is acting as supreme pastor and delivering a definitive judgement, he can hardly wish that his teaching should not be accepted. But Pope Paul did explicitly remind Christians of the duty to accept his teaching. To married people, he wrote, “Let married couples, then, face up to the efforts needed, supported by the faith and hope which do not disappoint . . . let them implore divine assistance by persevering prayer; above all, let them draw from the source of grace and charity in the Eucharist.’ (para. 25). To priests, he wrote, “Be the first to give, in the exercise of your ministry, the example of loyal internal and external obedience to the teaching authority of the Church” (para. 28). To his brother bishops: “At the head of the priests, your collaborators, and of your faithful, work ardently and incessantly for the safeguarding and the holiness of marriage, so that it may always be lived in its entire human and Christian fullness.” Pope Paul wished that his clear and definite teaching be embraced by all the faithful.
Read Fr. Crean's entire essay. And here is the Table of Contents for the current issue of the Quarterly.