In To Kill a Mockingbird, author Harper Lee paints ideas of social justice by showing its antithesis--social injustice. Social injustice is particularly expressed through the theme of racism pertaining to Tom Robinson.
Even before the trial begins, Atticus knows it is unlikely he will be able to convince the jury to acquit Robinson. Atticus first reveals his feelings of hopelessness to Uncle Jack in Chapter 9 when he admits that all he really has is Robinson' word against the Ewells' words. Atticus further implies he knows Robinson will be tried unfairly due to racism because, as he phrases it, "[R]easonable people go stark raving mad when anything involving a Negro comes up."
Social injustice is also revealed when the characters discuss the verdict of the jury. In Chapter 21, Reverend Sykes informs the children that, no matter how much the evidence points to Bob Ewell as the guilty culprit, Reverend Sykes has never once seen a jury rule in favor of a black man over a white man. Plus, Jem points out that the jury did not have to sentence Robinson to death even after determining guilt; they could have sentenced him to 20 years in jail instead. Atticus further points out the injustice of the racial judicial system in the following:
Tom Robinson's a colored man, Jem. No jury in this part of the world's going to say, "We think you're guilty, but not very," on a charge like that. It was either a straight acquittal or nothing. (Ch. 23)
All of these revelations concerning the injustice of the Southern judicial system point to Lee's ideas that social justice can only be built upon ideas of mutual equality and mutual respect for fellow human beings.