they accept the Marxist historic materialism as true and agree that capitalism is therefore a problem that needs to be adressed.

Basically they take what the bible says about a duty to care for the poor and then go to Marx to ask why are the poor poor and what can be done about it

to quote one of the influential early theologians in the movement

"In liberation theology, Marxism is never treated as a subject on its own but always from and in relation to the poor. Placing themselves firmly on the side of the poor, liberation theologians ask Marx:'What can you tell us about the situation of poverty and ways of overcoming it?' Here Marxists are submitted to the judgement of the poor and their cause, and not the other way around.

Therefore, liberation theology used Marxism purely as an instrument. It does not venerate it as it venerates the gospel. And it feels no obligation to account to social scientists for any use it may make - correct or otherwise of Marxist terminology and ideas...To put it in more specific terms, liberation theology freely borrows from Marxism certain 'methodological pointers' that have proved fruitful in understanding the world of the oppressed"