Will Willimon points us to this from MLK:

The contemporary church is often a weak, ineffectual voice with an uncertain sound. It is so often the arch supporter of the status quo. Far from being disturbed by the presence of the church, the power structure of the average community is consoled by the church’s silent and often vocal sanction of things as they are.
— Dr. Martin Luther King Jr., “Letter from Birmingham Jail,” in A Testament of Hope: The Essential Writings of Martin Luther King, Jr., edited by James Melvin Washington (San Francisco: Harper and Row, 1986), pg. 300.

Source: A Peculiar Prophet