The other answers have covered this topic well, but I can provide a few more quotes about Tom Robinson, justice, and equality.
First, after the trial, Mr. Underwood, the Maycomb newspaper editor, speaks out angrily about the injustice done to Tom Robinson in a newspaper editorial in which he states, in Scout's summation of his ideas,
Mr. Underwood didn’t talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom’s death to the senseless slaughter of songbirds by hunters and children.
Mr. Underwood calls out the murder of Tom Robinson as a slaughter of an innocent man—a sin.
The hypocrisy and injustice of the white townspeople supporting putting Tom Robinson in his "place" while criticizing Hitler's treatment of the Jews comes clear in Scout's concerns about her teacher, Miss Gates. Scout says to Jem,
Well, coming out of the courthouse that night Miss Gates was—she was goin‘ down the steps in front of us, you musta not seen her—she was talking with Miss Stephanie Crawford. I heard her say it’s time somebody taught ’em [the blacks] a lesson, they were gettin‘ way above themselves, an’ the next thing they think they can do is marry us. Jem, how can you hate Hitler so bad an‘ then turn around and be ugly about folks right at home—
In his final argument defending Robinson, Atticus appeals to the jury to consider the notions of equality on which the United States was founded:
We know all men are not created equal in the sense some people would have us believe—some people are smarter than others, some people have more opportunity because they’re born with it, some men make more money than others, some ladies make better cakes than others—some people are born gifted beyond the normal scope of most men. “But there is one way in this country in which all men are created equal—there is one human institution that makes a pauper the equal of a Rockefeller, the stupid man the equal of an Einstein, and the ignorant man the equal of any college president. That institution, gentlemen, is a court.
Atticus says in the above quote that equality before the law is the foundation of justice in the United States, and he asks the jury to respect this ideal.