A recent study found that a quarter-century ago, Brazil was 90% Catholic, but today it is 65%. There's also a rising cohort of secular Latin Americans with no religious affiliation, especially among youths and city-dwellers.
Moreover, of the 21 nations usually reckoned as part of Latin America, 14 of them are led by center-left governments that have sometimes crossed swords with the region's Catholic leaders over issues such as abortion and same-sex marriage -- including in the pope's home nation of Argentina.
Brazil has also recently been gripped by an anti-establishment mood, fueled by anger over spending on mega-events such as the 2014 soccer World Cup and the 2016 Olympics, while many ordinary people believe that services such as education, health care and transportation languish.
There's little indication that protesters want to embarrass the pope. Instead, they seem to be hoping to take advantage of his moral authority to bring attention to their cause. This week, one group that helped kindle the massive June demonstrations has plans for a rally under the banner "Pope, look how we're treated!"
The greater danger for Francis may be that all sides in the country's tensions may want to spin his message their way, especially with one eye on presidential elections in Brazil scheduled for next year.
If journalists aboard the papal plane today were hoping to draw Francis into a discussion of those challenges, they came away disappointed.
Francis walked back to the press compartment shortly after takeoff and spoke for only five minutes, focusing on the risks of a "throwaway society" that neglects both its youth and its elderly.
"I don't give interviews," the pope said by way of explanation.
"Why, I don't know, but I can't ... It's a little difficult for me, but I'm grateful for your company," he said.
Yet part of Francis' charm is that this skittishness didn't come off as a snub, because the pope proceeded to spend the better part of the next hour standing in the front of the economy cabin of the papal plane to personally greet each of the roughly 70 journalists on board.
Monday afternoon, Francis was scheduled to meet Brazilian President Dilma Rousseff at the Presidential Palace, and then the 76-year-old pontiff will rest on Tuesday before heading north to the famed Brazilian Marian shrine of Aparecida on Wednesday.
CNN's Barbara Arvanitidis contributed to this report.
John L. Allen Jr. is CNN's senior Vatican analyst and a senior correspondent for the National Catholic Reporter.