"They're certainly entitled to think that, and they're entitled to full respect for their opinions... but before I can live with other folks I've got to live with myself.  The one thing that doesn't abide by majority rule is a person's conscience." - Atticus Finch

During Atticus's conversation with Scout, They begin to talk about how everyone in the entire town resents Atticus for his actions in regards to the Tom Robinson case. Their disapproval was obvious through their shrewd comments, and Scout couldn't understand why Atticus thought that he was right and everyone else was wrong. It was then that he said to her what is written above. 

A man's conscience is the one thing that he can always count on to guide him.

This quote goes beyond the common lesson of "everyone is entitled to their opinion" and expands into another lesson entirely. This quote encompasses a rare but valuable message: never conform just for the sake of conformity. No matter how many people are doing it, if something is wrong, it's wrong. And if you know it is wrong, then you must resist it, no matter who is doing it or who tries to convince you to do it. The one thing that you can always count on to guide you to the right path is your conscience, because it is "the one thing that doesn't abide by majority rule".