What is the thematic significance of the quote from Rahim Khan in Hosseini's The Kite Runner, “There is a way to be good again”?

In The Kite Runner (Hosseini), one of the most powerful themes of the story is set forth in the simple statement of Rahim Khan, the old family friend of Baba and Amir, "There is a way to be good again" (2).  Amir must live with the guilt of his sins against Hassan, amongst these a failure to rescue his friend from being bullied and raped by Assef and his despicable act of framing Hassan for theft.  He has no opportunity for redemption, even supposing he had been capable of it.  He and Baba are forced to flee Afghanistan, and Ali and Hassan, who have left Baba's house, are left behind to face the chaos that Afghanistan has become.  Amir, in his dealings with Hassan, has not been good.

What Khan is offering is a chance at redemption.  That is the overarching theme of the novel. In order to be redeemed from one's sins, at the most basic level one must atone and be good again, trying to make up for what one has done.  Amir cannot bring back Ali and Hassan, who have died.  Nor can he make up for all of the sorrows they must have endured, but he can face down the enemy he so cowardly ran from, Assef, and he can rescue Sohrab, Ali's son and, as he learns, his nephew, from the clutches of the Taliban.  The reader knows nothing of this at the beginning, but the more we read of the story, the more we come to understand Khan's wisdom in offering this chance to Amir.

Khan plays a very quiet role in the story, but he is best positioned to understand Amir and how damaged Amir's life has been, by forces beyond his own control and by his own behavior.  Khan is the one who knows that Hassan is the child of Baba.  Khan is the one who sees how easily Amir lords it over Hassan because Amir is from a wealthy Sunni Pashtun family while Hassan is a mere servant who is a Shi'a Hazara, part of an ill-treated minority.  Khan is the one who knows that love is not meant to stop at an ethnic or religious line because he himself loved a Hazara woman.  Khan understands that Amir has good in him, but fears that he will never be whole without some form of redemption.  To be good again implies that there was a time when Amir had been a good person, and Khan believes he can be one again.