Martin Luther King Jr. uses figurative language strategically throughout his letter to promote equality. How does the following instance help accomplish his purpose of challenging the church leaders to take action against racial injustice?

Like a boil that can never be cured as long as it is covered up but must be opened with all its pus-flowing ugliness to the natural medicines of air and light, injustice must likewise be exposed, with all of the tension its exposing creates, to the light of human conscience and the air of national opinion before it can be cured.