Bishop Martin Mtumbuka of Malawi pulled no punches when passing judgement on the Vatican's stunning declaration that Catholic clergy could bless couples living in "irregular relationships," such as same-sex unions.

This "looks to us like a heresy, it reads like a heresy, and it affects heresy," he said. "We cannot allow such an offensive and apparently blasphemous declaration to be implemented in our dioceses" in southeast Africa.

The Fiducia Supplicans ("Supplicating Trust") document triggered debates around the world, but negative reactions have been especially strong in Africa, with strong protests from bishops' conferences in Malawi, Zambia, Cameroon, Ivory Coast, Rwanda, Angola and other nations.

"The Church of Africa is the voice of the poor, the simple and the small," wrote Cardinal Robert Sarah of Guinea, the former head of the Vatican's Congregation for Divine Worship and the Discipline of the Sacraments. "It has the task of announcing the Word of God in front of Western Christians who, because they are rich, equipped with multiple skills in philosophy, theological, biblical and canonical sciences, believe they are evolved, modern and wise in the wisdom of the world."

Cardinal Sarah endorsed the declarations from African bishops and added: "We must encourage other national or regional bishops' conferences and every bishop to do the same. By doing so, we are not opposing Pope Francis, but we are firmly and radically opposing a heresy that seriously undermines the Church, the Body of Christ, because it is contrary to the Catholic faith and Tradition."

These tensions resemble doctrinal fault lines seen during the 2015 Synod of Bishops on the Family, noted historian Philip Jenkins, the author of "The Next Christendom: The Coming of Global Christianity" and "Fertility and Faith: The Demographic Revolution and the Transformation of World Religions" and many other books.

"Religious faith and fertility are linked and it's easy to see that around the world," said Jenkins, reached by Zoom.

Catholic battles over Fiducia Supplicans, for example, echo decades of conflicts in the Anglican Communion. This past year, leaders of the Global Anglican Future Conference (GAFCON) and the Global South Fellowship of Anglican Churches rejected a General Synod vote allowing Church of England priests to offer blessing rites to same-sex couples.

GAFCON leaders said they "no longer recognize the Archbishop of Canterbury as an Instrument of Communion, the 'first among equals' of the Primates." Recent archbishops of Canterbury have, they added, "failed to guard the faith" by embracing bishops "who have embraced or promoted practices contrary to Scripture."

Meanwhile, more than 7,000 United Methodist congregations have left the denomination after decades of fighting about biblical authority and sexuality. But those departures are not the end of this story.

Membership in Africa is growing (fast) while the U.S. church is shrinking (fast)," noted the Rev. Chris Ritter of Geneseo, Illinois, writing in the national "People Need Jesus" newsletter. The General Conference "has never been more sharply divided. U.S. institutionalists have the majority to change church teaching on human sexuality, but they seem not to have the super-majority" needed for a compromise that would change teachings in some regions, but not others.

Thus, Ritter stressed: "U.S. institutionalists have their finger on a red button that will cause an African implosion."

Looking ahead, Jenkins stressed that it's important to recognize that parts of the Global South are also "becoming more complex" in terms of falling fertility rates and rising secularism. In Latin America, for example, Argentina, Brazil, Uruguay, Chile and parts of Mexico are "going through these changes very fast. … There isn't one Global South. There are now multiple 'Souths.' "

But clashes between growing churches and declining churches are also caused by changes in the North, he said. "Over the past 10 to 20 years, matters have accelerated so much in terms of demographic decline in the Global North as a fundamental difference with large parts of the Global South." This has created radically different trends in terms of the numbers of new clergy, children and converts in various religious groups -- including Catholicism.

Catholic debates over LGBTQ+ issues are crucial, he said, "because if you want to spot low-fertility, low-faith cultures in Europe and elsewhere, you look at how and when they legalized and legitimized same-sex marriage. That will give you a good idea of what is happening. … Just look for large numbers of secular old people."