Pope Francis has criticised the Brazilian church's failure to keep its flock from straying to evangelical churches, challenging the region's bishops to be closer to their people to understand their problems and offer them credible solutions.

In the longest and most important speech of his four-month pontificate, Pope Francis drove home a message he has emphasised throughout his first international trip at World Youth Day: the need for priests and young Catholics to shake up the status quo, get out of their stuffy sacristies and reach the faithful on the margins of society or risk losing them to rival churches.

Pope Francis took a direct swipe at the "intellectual" message of the church that so characterised the pontificate of his predecessor, Pope Benedict XVI.

He said ordinary Catholics simply do not understand such lofty ideas and need a simpler message of love, forgiveness and mercy.

"At times we lose people because they don't understand what we are saying, because we have forgotten the language of simplicity and import an intellectualism foreign to our people," he said.

"Without the grammar of simplicity, the church loses the very conditions which make it possible to fish for God in the deep waters of his mystery."

In the speech outlining the kind of church that this new pope wants, Pope Francis asked bishops to reflect on why hundreds of thousands of Catholics have left for charismatic Pentecostal congregations that have grown exponentially in recent decades, particularly in Brazil's slums or favelas, where their charismatic message and nuts-and-bolts advice have been welcomed by the poor.

According to Brazilian census data, the number of Catholics dipped from 125 million in 2000 to 123 million in 2010, with the church's share of the total population dropping from 74% to 65%.

During the same period, the number of evangelical Protestants and Pentecostals has risen from 26 million to 42 million, an increase of 15% to 22% of the population in 2010.

Pope Francis offered a breathtakingly blunt list of explanations for the demographic shift.

"Perhaps the church appeared too weak, perhaps too distant from their needs, perhaps too poor to respond to their concerns, perhaps too cold, perhaps too caught up with itself, perhaps a prisoner of its own rigid formulas," he said.

"Perhaps the world seems to have made the church a relic of the past, unfit for new questions. Perhaps the church could speak to people in their infancy but not to those come of age."

Pope Francis asked if the Catholic Church of today still was able to "warm the hearts" of its faithful, if its priests took the time to listen to their problems and remain close to them, and act like a "mother" who not only gives birth to her children but cares for them.

"We need a church capable of rediscovering the maternal womb of mercy," he said.

"Without mercy, we have little chance nowadays of becoming part of a world of 'wounded' persons in need of understanding, forgiveness and love."

The Vatican said Pope Francis read the five-page speech in its entirety to the 300 or so bishops gathered for lunch in the auditorium of the Rio archbishop's residence.

He will issue a similarly lengthy and important speech tomorrow to the bishops of Latin America, said the Fr Federico Lombardi, the Vatican spokesman.

The Argentine pope began his day with a Mass in Rio's beehive-like modern cathedral where he exhorted 1,000 bishops from around the world to go out and find the faithful, a more diplomatic expression of the direct, off-the-cuff exhortation he delivered to young Argentine pilgrims on Thursday.

In those remarks, he urged the youngsters to make a "mess" in their dioceses and shake things up, even at the expense of confrontation with their bishops and priests.

He also attended a meeting with Brazil's political, economic and intellectual elite, urging them to look out for the poorest and use their leadership positions to work for the common good.

He also called for greater dialogue between generations, religions and peoples.

He delivered those remarks at Rio's grand municipal theatre, where he was welcomed with a standing ovation and shouts of "Francisco" and "Viva o Papa!" (Long live the pope).