Atticus allowed a Cunnigham to serve on the jury. Atticus believed that during that night when Scout, Jem, and Dill showed up at the jail, the Cunningham clan grew a new respect for the Finches.
During that evening, Mr. Cunningham was humbled by Scout's understanding of his situation. She also helped him understand that she was friends with his son. Bringing his son into the discussion helped Cunningham see that Atticus too was a parent of young children just trying to teach them right from wrong. Mr. Cunnigham understood human kindness that night.
Atticus believed a Cunningham could understand human kindness again by the time the trial took place. It was blatantly obvious to anyone watching that Atticus' case was clear: Tom was innocent. Although all the other jurors did the right thing according to the racial expectations of the time, one juror took considerable wearing down. Atticus believes this was his Cunningham friend.