To amplify: where most theologians
attempt to frame Christian belief in a philosophical
context, liberation theologians frame their beliefs in the context of sociology--specifically Marxist
John Paul II isn't the primary opponent of liberation theology--rather Joseph Cardinal Ratzinger, the chief theologian for the Vatican, has led the charge against liberation theology. His attempt to squash the movement entirely in the mid-80's was actually cut off by the Pope after complaints from Brazilian bishops. Ratzinger, like other critics of liberation theology, claims that Marxism is inherently anti-Christian and that liberation theologians unwittingly allow themselves to be duped by Marxist ideology, which supposedly warps Christian beliefs into a merely political system.
Ratzinger is wrong--liberation theologians have done a good job of adopting Marx's social analysis without taking on his commitment to atheism. Their problem is that Marx's analysis of society was substantially wrong. Marx is about as relevant to economics as Freud is to psychology--he's historically very important and he was the first person to grasp some key concepts, but the details of his theory have been largely disproven. Thus, liberation theologians who use Marxism as their framework are destined to make mistakes--not because they've abandoned Christianty, but because they're using outmoded social science as the basis for their system. This produces a new kind of bad theology that is no less damaging that the usual type.