"I want you to listen to me very carefully, Harry. You're not a bad person. You're a very good person, who bad things have happened to. You understand? Besides, the world isn't split into good people and Death Eaters." - Sirius Black to Harry Potter, 'Harry Potter and the Order of the Phoenix'.
What I love most about this quote from Sirius is that I can imagine Remus Lupin saying it to him during their time at Hogwarts.
Imagine: Sirius has just abandoned his family and finally run away, and is living with James and the Potter family. But before he did, his mother had screamed:
"You are a Black, Sirius - you have our blood in you. It runs deep inside you. You are one of us. You always will be."
And Sirius begins to believe it. He starts to believe that he is, too, a bad person. Like his family. And he tells the Marauders how he feels, and he is vulnerable in front of them for one of the first times in his life. And Remus shuffles forward onto Sirius' bed, kneels in front of him and touches his face and runs his finger across his cheek, wiping the tears and, very simply, tells him:
"I want you to listen to me very carefully, Padfoot. You're not a bad person. You're a very good person, who bad things have happened to. You understand? Besides, the world isn't split into good people and bad people. You're a Marauder. And we love you."
AND - I can also imagine Sirius having said it to Remus in return, after a particularly nasty transformation. Remus cries that he is afraid that he, deep down, is a bad person. And Sirius shuffles closer to him, placing a hand on his cheek and running his finger softly down his scar before saying:
"I want you to listen to me very carefully, Moony. You're not a bad person. You're a very good person, who bad things have happened to. You understand? Besides, the world isn't split into good people and bad people. You are my Moony. And I love you."
And Sirius remembers it and holds the words close to him. And then finally shares them with Harry, because that's the thing that got him through the torture of Azkhaban and the Dementors.