In 1968, attending the Conference of Latin American Bishops in Medellín, Colombia, he endorsed the fundamental principle of liberation theology, the "preferential option for the poor". In 1984, he joined other Brazilian prelates in Rome when theologian Leonardo Boff, the foremost figure in the liberation theology movement and a former student of Arns, was examined by Cardinal Joseph Ratzinger (later Pope Benedict XVI), head of the Congregation for the Doctrine of the Faith. Boff said that he thought he was not actually the target of the Vatican investigation as much as the entire church in Brazil and its activism on behalf of the poor. Arns predicted that Boff's examination would produce no "surrender" because "The liberation of the poor is an aspiration rooted in human dignity. The message of liberation is central to Christianity." One historian described it as "not an exercise in abstruse theological semantics but a debate over the future of the Church in Brazil." Arns and Cardinal Aloisio Lorscheider of Fortaleza joined Ratzinger and Boff for part of their four-hour meeting, after being denied their request to attend the entire meeting. Later meetings between Brazil's senior prelates, including Arns, and Pope John Paul II, cooled the conflict to a degree, and in 1986 Arns offered a conciliatory statement that he agreed with the Pope's admonition against priests taking part in politics directly, but he defended the church's advocacy on behalf of such powerless groups as peasants and native peoples, workers and inhabitants of urban slums.