[Editor’s note: This is the first in a series Crux pieces to come marking Humanae Vitae’s anniversary.]

This Sunday marks the 50th anniversary of the presentation of the seventh, and final, encyclical of Pope Paul VI, Humanae Vitae. Perhaps the most contested papal document in modern history, it was both applauded and bitterly criticized for boldly upholding Church teaching on contraception in the era of the sexual revolution.

Called “On the regulation of birth” in English, but better known by its Latin title, the document struck the late 1960s like an earthquake— so much so that Paul VI, clearly stung by the widespread negative reaction, didn’t publish another encyclical for the remaining ten years of his pontificate.

Tremors both within and outside the Church triggered by Humanae Vitae are still being felt today.

Still today, even outside Church boundaries, there are political and social forces demanding that the current pope renounce the encyclical’s teaching. Two weeks ago, for instance, Great Britain’s International Development Secretary visited the Vatican and urged Church officials to make it easier for young girls to have access to contraception.

RELATED: UK government minister urges Vatican to change stance on birth control

On the inside, dissenting voices within Catholicism also abound, with bishops, priests and laypeople often calling either for a change in the teaching, or flat-out ignoring it in the confessional. A 2014 poll from Univision, for instance, showed that 79 percent of Catholics reject the Church’s teaching on artificial contraception.

But there’s nothing new here: The opposition dates all the way back to the year of the publication of Humanae Vitae, when seven conferences of Catholic bishops from around the world more or less challenged it.

Ferment, then and now

Leading the charge were the Canadian bishops, who in September of 1968, two months after the encyclical was released, produced a document known as the ‘Winnipeg Statement.’

It has been widely interpreted as providing a loophole whereby Catholics may feel permitted to use birth control, as it says that “a certain number of Catholics” find it “either extremely difficult, or even impossible, to make their own all elements of this doctrine.”

Central to the debate, according to the Canadian bishops, was the role and importance of personal religious freedom of conscience.

The Canadians weren’t alone. The Belgian bishops wrote that anyone who, with a “well-founded judgement,” comes to a conclusion on contraception different from that presented in the encyclical should not “be regarded as an inferior Catholic.”

Fast forward 25 years from the publication of Humanae Vitae to the silver anniversary of the encyclical, and the contrasting voices within the Church were still going at it.

In May of 1992, in a debate published by the Italian monthly periodical Jesus with then-Cardinal Joseph Ratzinger, at the time head of the Vatican’s doctrinal office, today Pope emeritus Benedict XVI, the late Cardinal Franz König of Vienna, Austria, famously described the encyclical as the “irritating distinction between artificial and natural contraception.”

“Here [on birth control], we have ended up in a bottleneck above all because of the distinction (cast into doubt even by medicine) between artificial and natural, as if even from the moral viewpoint what is important is the trick of cheating nature,” König wrote.

Yet for every act of defiance against Humanae Vitae that’s come out since 1968, there’ve been others in favor. Nowhere to be found online today, but still representative of the upheaval, is a rousing four-page defense of the document released by the bishops of Scotland in October 1968.

They begin by addressing the “surprise and confusion” caused by the criticism, and opposition the document had met with among the faithful.

“There can be no doubt that the pope speaks on this occasion as the Vicar of Christ and in virtue of his office as supreme pastor and teacher,” the Scottish bishops write. “Therefore, his teaching calls for acceptance.”

The prelates acknowledged the role of conscience, and that people have the “right and duty to follow” it. Yet “man is not a law to himself; his conscience is not completely independent. The role of conscience is indeed to judge whether our actions are morally good or bad. But these judgements must be based on sound principles of right and wrong.”

And the debate goes on.

On the one hand, American prelates such as Cardinal Donald Wuerl of Washington and Archbishop Jose Gomez of Los Angeles have recently come out defending Humanae Vitae, with the latter saying it should be read as a “prophecy” and a “promise” on Twitter.

In a series of tweets, Gomez said that looking back one can understand how the world “was not ready” for Humanae Vitae in 1968, as it was a “time of new ideas about human freedom and love and new attitudes toward traditions and authority.”

According to Gomez, much of what Paul VI warned of 50 years ago subsequently has happened: “From rampant divorce, infidelity and pornography, to test-tube babies, widespread abortion, ‘demographic winter,’ and the total confusion about gender, sexuality and the human person that we see in our society today.”

Gomez then moves on to say that the world didn’t know how much it needed the encyclical, nor how much more it’s needed today.

It’s also a promise, he argued towards the end of his thread, saying that the document is a “letter about happiness and love. Blessed Pope Paul writes of ‘God’s loving design’ — the path he sets before us that will lead us to find happiness. Married love is a part of that plan.”

Wuerl addressed the anniversary of the signing of the document – marked July 25, though it was released July 29 – with a blog post.

“In this modern age when sexual activity is often seen as recreational and without consequence, the message of Humanae Vitae is a sign of contradiction to the world and is challenging for some,” he wrote.

Yet, he added, “it goes back to our basic understanding of the dignity and role of human beings, male and female, complementary and equal, in God’s plan.”

“Moreover, we have the promise of the Spirit of truth, and so we can place our faith and walk with moral certitude in the teaching of Peter the Rock on which stands the Church,” Wuerl said.

In his post, Wuerl goes through the comments made by Paul VI’s immediate successors, up until Benedict, all in support of Humanae Vitae and its teaching.

Though today it’s rarer to hear cardinals or archbishops openly challenging the document, that’s not to say the critics among the hierarchy have disappeared, according to Father Tom Reese, a Jesuit, who recently wrote about it at Religion News Service.

Noting that many today say the document was prophetic in its conviction that “contraceptives led to the separation of sex from procreation and therefore to conjugal infidelity, disrespect for women, gender confusion, and gay marriage,” Reese said that the controversy was never about the document as a whole, but the ban of “every single use of artificial contraception.”

Implying that contraception caused all of these “problems,” he argued, “is absurd, an insult to all the good people who have used contraceptives at some point in their lives.”

According to Reese, “It is probably impossible for [the Church] to simply admit it was wrong. The Church is not very good at that.” Yet it could say that abortion “is a far greater evil, and anyone who might be tempted to have an abortion should practice birth control.”

In October 2014, when he was celebrating the beatification Mass for Paul VI, Francis avoided making a direct comment on Humanae Vitae, yet many observers highlighted a phrase from his homily as a clear reference: “Facing the advent of a secularized and hostile society, [Blessed Paul] could hold fast, with farsightedness and wisdom – and at times alone – to the helm of the barque of Peter.”

Francis was more direct in the Philippines back in 2015, as he addressed hundreds of families at a local sports arena.

“At a time when the problem of population growth was being raised, [Paul VI] had the courage to defend openness to life in families,” Francis said. “He knew the difficulties that are there in every family, and so in his Encyclical he was very merciful towards particular cases, and he asked confessors to be very merciful and understanding in dealing with particular cases.”

“But he also had a broader vision: he looked at the peoples of the earth and he saw this threat of families being destroyed for lack of children. Paul VI was courageous; he was a good pastor and he warned his flock of the wolves that were coming,” the pontiff said.

In what might be taken as a further sign of ratification, Francis is set to declare Pope Paul VI a saint in October during a Synod of Bishops on youth.