One of the most perceptive books ever written about the Second Vatican Council was Ralph Wiltgen’s 1967 “The Rhine Flows into the Tiber,” about how many of the reform energies at the council first took shape in German-speaking Catholicism.

If someone were to write a book about the 2014 Synod of Bishops on the family, a terrific title along the same lines might be: “The Rhine Flows into the Tiber Again … and Hits the Zambezi.”

Just as in Vatican II, some of the leading voices at the 2014 synod pushing a moderate-to-progressive agenda are Germans. The difference is that this time they’ve met stiff resistance from several African bishops, who no longer regard themselves as junior partners in Catholicism Inc. This time, they’re ready for the board room.

Of course, not all Germans or Africans, inside the synod or out, think alike. Still, the contrast between the two camps has been fairly unmistakable.

Retired German Cardinal Walter Kasper is the leading proponent of the permissive position on the issues of giving Communion to Catholics who divorce and remarry outside the Church, and some of the most articulate figures advocating a more compassionate tone overall on sexual morality include Cardinal Reinhard Marx of Munich and Cardinal Christoph Schönborn of Vienna, Austria.

By the same token, several of the most forceful leaders on the other side of the argument come from Africa.

After Monday’s release of an interim synod report created impressions that the Catholic Church was softening its traditional opposition to same-sex unions and other “irregular” relationships, it was Cardinal Wilfrid Fox Napier of South Africa who went on the offensive during a Vatican briefing.

“The message has gone out that this is what synod is saying, this is what the Catholic Church is saying, but that’s not what we’re saying,” Napier insisted.

“It’s not true … that this synod has taken up these positions,” he said, arguing that the interim report was not approved by the entire synod.

Napier has been using his Twitter account throughout the synod, for instance saying Oct. 10 that “development” in Catholic teaching, which some use to justify more positive stances on same-sex unions and other matters, is not the same thing as changing it.

Cardinal Robert Sarah of Guinea, who heads the Vatican’s charity department Cor Unum, has been another outspoken voice.

“Based on the Sacred Scriptures, the tradition of the Church has always stated that ‘the acts of homosexuality are intrinsically disordered, since they are against the natural law, and preclude the gift of life,’ ” he said in an Oct. 16 interview with the Catholic News Agency.

“They cannot be approved in any case,” Sarah said, basically quoting official Church teaching.

Sarah recalled a phrase of Pope John Paul II, who once asked if the push for same-sex unions is “part of a new ideology of evil.”

Perhaps the most forceful African statement so far came from Nigerian Archbishop Ignatius Kaigama, who told a Vatican briefing Oct. 8 that some international organizations try to coerce African nations to adopt a liberal sexual ethic by making it a condition of development assistance.

“We get international organizations … which like to entice us to deviate from our cultural practices and traditions and even religious beliefs,” he said. “This is because they believe that their views should be our views, their opinions and concept of life, should be ours.”

“We say no,” Kaigama said. “We have come of age.”

“Most countries in Africa have been independent for 50, 60, even 100 years, and we should be allowed to think for ourselves,” he said. “The time has gone when we would just follow [the West] without question.”

The African ferment has been clear in multiple ways over the past two weeks.

For instance, when Pope Francis hand-picked a slate of six prelates to shape the synod’s final document, Africans objected that no one from their continent had been selected, so Francis added Napier on Thursday.

Also yesterday, Kasper gave a brief interview outside the synod hall in which he said the Africans “should not tell us too much what we have to do.”

(There was brief fracas when Kasper denied giving the interview and declared himself shocked, then the journalist who conducted it, Edward Pentin, posted the audio file.)

In reality, whatever one thinks of Kasper’s position on divorce and remarriage, he’s a true gentleman who was speaking a second language toward the end of a very grueling couple of weeks, and almost certainly didn’t mean offense. In context, he seemed to be saying that different parts of the world have different problems and should be allowed to develop their own solutions.

Yet the fact that the comment got spun up into a “Kasper v. Africans” melee shows just how firmly the Africans are on the radar screen here.

The boldness of the African participants reflects broader trends in Catholic life, especially the demographic shift in the Church’s center of gravity from north to south over the late 20th and early 21st centuries.

During the last century, the Catholic population of sub-Saharan Africa went from 1.9 million to more than 130 million – a staggering growth rate of 6,708 percent. Vocations are also booming across the continent. Bigard Memorial Seminary in southeastern Nigeria, with an enrollment of more than 1,100, is now said to be the largest Catholic seminary in the world.

Those numbers have helped give African Catholics a sense that their historical moment to lead has arrived.

This rise of Africa offers a gut check for both the Catholic left and the Catholic right in the West, in different ways.

Liberals are supposed to be all in favor of empowering the “Third World” and hearing their voices, but that tends to end when those voices say things liberals don’t want to hear, especially on sex and the family.

Conservatives cheer African Catholics who line up with them on the culture wars, but they often fidget when the conversation turns to other matters. On American foreign policy, for instance, or the ethical failures of free-market capitalism, or immigrant rights, the Africans often stake out positions that would be considered almost radically leftist in American terms.

No matter what one makes of what the Africans have to say, here’s an empirical fact of Catholic life brought into clear focus by the 2014 synod: Get used to it, because they’re not going away and they’re not going quiet.