Atticus Finch is an honest man because he acts on his conscience rather than what others expect.
Throughout the story, Atticus Finch catches a lot of grief for following his conscience rather than acting for convenience. He not only represents Tom Robinson, he actually tries to mount a defense. Most lawyers would simply put in time but not really try, either because they did not want to represent a black man or because they knew they would lose. Atticus actually tried to win.
Atticus explained to his daughter Scout why he could not give up, even though he knew he would lose before he started.
"Simply because we were licked a hundred years before we started is no reason for us not to try to win," Atticus said. (ch 9)
Atticus tells her he could not hold his head up if he did not do his best. He could not tell his kids how to behave or go to church if he did not try.
Atticus is respected by the community, even some of the whites, because of this trait. The people know what Atticus is a stand-up guy. He will do what is right, no matter the cost. Another example is when he shot the rabid dog, even though he had not shot a gun in years. Atticus is a good man, and everyone knows it. Some people disagree with his choice to defend Tom Robinson, but this is partially because they know that he will actually try.