SAO PAULO, Brazil — In the early 1980s, when Pope John Paul II wanted to clamp down on what he considered a dangerous, Marxist-inspired movement in the Roman Catholic Church, liberation theology, he turned to a trusted aide: Cardinal Joseph Ratzinger.

Now Cardinal Ratzinger is Pope Benedict XVI, and when he arrives here on Wednesday for his first pastoral visit to Latin America he may be surprised at what he finds. Liberation theology, which he once called "a fundamental threat to the faith of the church," persists as an active, even defiant force in Latin America, home to nearly half the world's 1 billion Roman Catholics.

Over the past 25 years, even as the Vatican moved to silence the clerical theorists of liberation theology and the church fortified its conservative hierarchy, the social and economic ills the movement highlighted have worsened. In recent years, the politics of the region have also drifted leftward, giving the movement's demand that the church embrace "a preferential option for the poor" new impetus and credibility.

Today some 80,000 "base communities," as the grass-roots building blocks of liberation theology are called, operate in Brazil, the world's most populous Roman Catholic nation, and nearly 1 million "Bible circles" meet regularly to read and discuss Scripture from the viewpoint of the theology of liberation.

During Pope Benedict's five-day visit here, he is scheduled to meet with President Luiz Inacio Lula da Silva, canonize a saint, preach to the faithful and visit a drug treatment center before addressing the opening session of a conference of Latin American bishops that will discuss the future of the church in the region where liberation theology originated, prospered and drew so much of his censure. Some liberation theology supporters will be present, others will be at a parallel meeting, and all have been cautioned not to be too aggressive in pressing their views.

In the past, adherents stood firm as death squads made scores of martyrs to the movement, ranging from Archbishop Oscar Arnulfo Romero of El Salvador, killed in 1980 while celebrating Mass, to Dorothy Mae Stang, an American-born nun shot to death in the Brazilian Amazon in February 2005. Compared to that, the pressures of the Vatican are nothing to fear, they maintain.

"Despite everything, we continue to endure in a kind of subterranean way," said Luiz Antonio Rodrigues dos Santos, a 55-year-old teacher active in the movement for nearly 30 years. "Let Rome and the critics say what they want; we simply persevere in our work with the poor and the oppressed."

On a cool and cloudy Saturday morning in late April, evidence of the movement's vitality was plain to see. Representatives of 50 base communities gathered at the St. Paul the Apostle Church on the east side of this sprawling city, an area of humble workers' residences and squatter slums.