These dramatic words are from Atticus Finch's closing argument in defense of his client, Tom Robinson- a black man falsely accused of raping and beating a white woman.
Atticus is one of the few people in Macomb who recognizes no cultural nor racial differences among people. As an educated man, he knows what weaknesses and strengths every individual possesses. As a learned man in life, he respects everyone equally.
With these words, Atticus is merely repeating the common notion of the people of Macomb:
"........that you gentlemen would go along with them on the assumption, the evil assumption, that all Negros lie, all Negros are basically immoral beings, all Negro men are not to be trusted around our women. An assumption that one associates with minds of their caliber and which is itself gentlemen a lie which I do not need to point out to you.
Therefore, what Atticus actually does is to reflect the mentality of the Macomb folk and expose it for what it is: A very ignorant, limited, and reduced view of the world which can only be inherent to ignorant, limited, and reduced people.
Hence, Atticus uses the views that Macomb has on black people, and turns those views back onto the people, themselves. As a result, even though the outcome is not rosy, at least the citizens of Macomb may have experienced for the first time a closer look on their ignorant perception of reality.