What message was Atticus trying to convey to his daughter at the end of the book when he said, "Most people are, Scout, when you finally see them"?

When Atticus said "Most people are, Scout, when you
finally see them," he was referring to Boo Radley, who had saved Scout's life earlier in
the evening when she and Jem were attacked by a vengeful and drunken Bob Ewell.  Boo had
pulled the angry Ewell off the kids, although not before Jem's arm had been broken, and
in the aftermath, was in the Finch household checking on Jem.  Scout, demonstrating how
much she'd grown up and absorbed the lessons of Southern womanhood administered to her
by Aunt Alexandra, had walked Boo home, and as she drifted off to sleep, mentioned to
her father that "He was real nice."  When Atticus said "Most people are, Scout, when you
finally see them," the statement pointed to the greater theme of the novel, that of the
impact of people's treatment--and mistreatment--of each other based on preconceived
notions, fears, and stereotypes.