Pope Benedict XVI last week began a five-day pontifical voyage to Brazil, his first trip to Latin America since becoming pope. Home to 124 million Roman Catholics, Brazil was also a crucible of "liberation theology", the bid by thousands of Catholic priests and nuns, especially in South America, to align themselves with the poor and to combine worship with political action aimed at transforming radically unjust societies.

What is Pope Benedict's attitude towards liberation theology?

As the Prefect of the Congregation for the Doctrine of the Faith . . . once known as the Inquisition . . . under Pope John Paul II, Cardinal Joseph Ratzinger, as he then was, singlehandedly eviscerated the liberation theology movement, silencing its most articulate voices. The late pope, with his long experience battling against official oppression in Poland, was instinctively disposed in its favour, describing the movement as "useful and necessary".

But Ratzinger, on whom Pope John Paul II depended for theological advice throughout his papacy, persuaded him that yoking the church to political liberation movements was a deadly danger. He invoked "totalitarian and atheistic regimes" . . . Nazi and communist . . . "which came to power by violent and revolutionary means, precisely in the name of the liberation of the people. This shame of our time cannot be ignored." These regimes, he went on, promised freedom but delivered slavery. "Those who. . . make themselves accomplices of similar enslavements betray the very poor they mean to help, " he insisted. He ruthlessly silenced its exponents, including the most prominent, a Brazilian priest called Leonardo Boff, who later left the church.

Where did liberation theology originate?

The movement had its origins in the Second Vatican Council in the 1960s and its exhortation to the church to show solidarity with humanity "and especially the poor". The message had a particular resonance for Latin America. In 1968 an assembly of bishops at Medellin in Colombia voted in favour of what they called "a preferential option for the poor", guided by Gustavo Gutierrez, theological adviser to the assembly and coiner of the phrase "liberation theology". At the heart of the movement was a desire to harness the energy of the church to the goals of social revolution.

"As Aquinas baptised Aristotle, " says Tina Beattie, the Catholic writer, "so liberation theologians sought to baptise Marx." But while the poorest regions of the world . . . it was also influential in Africa . . . were the places where one would expect liberation theology to resonate and gain followers, Ratzinger insisted, "It is not a homegrown [ie Latin American] product but a European export" . . . the pernicious invention of European Marxists.

Why was Ratzinger so intolerant of it?

The Bavarian theologian began his career as a liberal but was profoundly affected by the student movements of the late 1960s, when the University of Tubingen, where he was teaching, became a centre of militancy. According to another member of the theological faculty, Hans Kung, the students "came in and occupied the pulpits. . . For someone timid like Ratzinger it was horrific."

Ratzinger's views then underwent a 180-degree shift and he became profoundly conservative. One of his students recalls that Ratzinger had been "very open, fundamentally ready to let in new things. But suddenly he saw these new ideas were connected to violence and destruction of the order that came before.

He was simply no longer able to bear it."

Has the pope since changed his ideas?

No. Years after the suppression of liberation theology in the 1980s, Ratzinger spoke of what he had done as his greatest achievement. In his eyes, the struggle against liberation theology continues, as the disciplining in March this year of Fr Jon Sobrio, a 69-year-old Spanish priest based in El Salvador and for years a leading figure in the movement, indicates.

Did the movement go away?

Some predicted it would be killed off by the fall of the Berlin Wall and the subsequent collapse of socialist regimes in Europe. But socialism never lost its relevance for Latin America, where capitalism goes hand in hand with feudal landlordism, the army and the Catholic church as an irresistible machine of oppression. The man who welcomes Pope Benedict to Brazil today, president Lula da Silva, is himself a product of the movement, and a friend and admirer of many in the movement. Leonardo Boff believes Ratzinger's mistake was to see Marxism as the church's great enemy in Latin America: "Here the problem was always savage capitalism which creates millions of poor. These poor shout for social change and justice. Liberation theology was born hearing those cries. It sought to be a social force to lift a crucified people out of misery. Condemning this theology, the Vatican weakened the fight of the oppressed and played the game of the perverse elites. This left the people sad because they said: how could the pope be on the side of our oppressors and not join our fight for liberation?"

What difference did the church's condemnation make?

Lula and other left-wing Latin American leaders might have faced less formidable challenges if Ratzinger had stayed his hand and allowed the church to continue backing their poor in their struggle against what the liberation theologists described as "structural sin" . . . the evil inherent in unjust societies, such as neocolonialism and devastating IMF-derived economic policies. Instead, as the Vatican expert John L Allen Jr puts it, thanks to Ratzinger, "The Catholic church in Latin America has played nothing like this transformative role." And it has seen its numbers in Brazil plummet, from 83% of the population in 1991 to 67% in 2005.

"The people no longer feel [the church] as their spiritual home, nor understand well its message, " Boff said this week.