How does Atticus explain his decision to have allowed Mr. Cunningham to sit on the jury in Harper Lee's To Kill a Mockingbird?

In Harper Lee's To Kill a Mockingbird, it was not actually Mr. Walter Cunningham who served on Tom Robinson's jury, but it was one of the Cunninghams' family members from Old Sarum. Atticus explains to Jem in Chapter 23 that he had decided not to strike the Cunningham from the jury because he had a hunch the Finches had earned the Cunninghams' respect the night of the lynch mob.

According to Atticus, the Cunningham on the jury was the one person who kept the jury out for hours without a decision, whereas normally the jury would have been back within a matter of minutes for a case like Robinson's. The Cunningham had at first wanted to acquit Robinson, but the other jury members convinced him otherwise. Atticus further explains that the reason why the Cunningham had wanted to acquit Robinson is because the Cunninghams are a very loyal clan:

Once you earned their respect they were for you tooth and nail. (Ch. 23)

Atticus had suspected he had earned the Cunninghams' loyalty the night of the lynch mob because Scout had reminded Walter Cunningham of his humanity, who then broke up the lynch mob. Scout had reminded Walter Cunningham of his humanity by speaking kindly of his son and by speaking compassionately of his entailment, telling him Atticus had once said he and Mr. Cunningham would face the entailment together and "ride it out together" (Ch. 16). After that, Mr. Cunningham had remembered just how much he is indebted to Atticus and how much he respects him; therefore, Mr. Cunningham had broken up the lynch mob, and his family member on the jury had become willing to defend Atticus by acquitting Robinson.