Published Wednesday in local media, the results of the "New Map of Religions," a census conducted by the Getulio Vargas Foundation (GVF), showed that from 2003 to 2009, the proportion of Brazilians who identify as Catholic dropped from 74 to 68 percent.
"That is a strong transformation rate. Changes that take place in 100 years are now taking place within ten. If this drop of one in 100 Catholics each year continues, in 20 years, less than half of the population will be Catholic," said Marcelo Neri, the head of GVF.
World Youth Day (WYD) is a youth-oriented international event for Catholics attended by the Pope. After being held in Madrid this year, the next WYD is scheduled for July 23 through July 28, 2013 in Rio de Janeiro.
Rio is Brazil's second least-Catholic region, with 49 percent claiming the religion. The only region behind Rio is northern Roraima State at 46 percent.
Brazil remains the most Catholic country in the world, with 130 million adherents, but this is the first time in 140 years that less than 70 percent of the population is Catholic.
According to Neri, the investigation also showed a "stagnation" of Evangelical Pentacostals, such as the Universal Church of the Kingdom of God or the Assembly of God.
Their numbers nearly doubled during the 1990s, especially among the poor. But from 2003 to 2009, their membership grew just slightly from 12.5 percent to 12.8 percent.
The study also revealed that those claiming they are "without religion" rose from 5 percent to 6 percent.