Latin America has 42% of the world’s Catholic population, a dominance that is not expected to change appreciably over the next few decades. Hence the Vatican must be viewing with concern the declining fortunes of Catholicism in the most populous country on the continent, according to an article in the New York Times.

Despite the iconic Christ the Redeemer statue that towers over this city, there is deep anxiety among some Catholics about the future of their faith, given rising secularization and indifference to religion here. Only 65 percent of Brazilians now say they are Catholic, down from more than 90 percent in 1970, according to the 2010 census. The decline has been so steep and continuous, especially in Rio de Janeiro, that one of Brazil’s top Catholic leaders, Cardinal Cláudio Hummes, has remarked, “We wonder with anxiety: how long will Brazil remain a Catholic country?”

What has happened to those lapsed Catholics? One problem is competition from evangelical and charismatic churches. Some Catholic churches have responded by adopting some of the practices of their competitors and have created mega-churches. But there is a deeper threat that cannot be combated by speaking in tongues and having priests behave like pop singers, and that is-you guessed it-secularism.

But while evangelicals have grown more powerful in Brazil, a new shift threatens churches of all stripes: the rise of secularism. Andrew Chesnut, an expert on Latin American religions at Virginia Commonwealth University, said that the fastest-growing segment in Brazil’s religious landscape may now be nonbelievers and people unaffiliated with any church, making up as much as 15 percent of the population.

For a country that as recently as 1980 had negligible levels of people saying they were atheists, this development points to big shifts in society. Compounding the problem for the Vatican, many people in Brazil who say they are Catholic rarely attend Mass, and practicing Catholics often express frustration with the Vatican’s policies.

Across Latin America, growing numbers of people say they have no religious affiliation, a phenomenon similar to what has happened in Europe and the United States, but somewhat less pronounced, said Philip Jenkins, a history professor who teaches at the Institute for Studies of Religion at Baylor University. One sign of this, experts say, is the drastic drop in fertility rates, which for the church means fewer children to be baptized and confirmed, fewer young candidates to become priests and nuns, and diminishing ties for Catholic parents to the church.

Brazil’s fertility rate, one of Latin America’s lowest at about 1.83 children per woman, is below the level needed to keep the population stable.

“If I were a Brazilian cardinal, I would be even more worried about family size and fertility rates, which are a very good augur of secularization, than Pentecostalism,” Dr. Jenkins said.

This is always going to be a problem for religion which maintains its numbers via a largely non-biological hereditary mechanism. It depends upon families producing children who will adopt the religion of their parents and thus provide a steady replenishing of the membership of churches. Modernity thus poses a double threat to religion: a rise in secular thinking and a drop in fecundity.