Amir's guilt over his cowardice in refusing to help Hassan when he is sodomized by Assef is so overwhelming that he commits an even greater sin by planting money under Hassan's mattress so he will be accused of theft. But Baba surprises him by forgiving Hassan.
Forgive? But theft was the one unforgivable sin... Then how could he just forgive Hassan? And if Baba could forgive that, then why couldn't he forgive me for not being the son he'd always wanted? (Chapter 9)
Amir will later discover Baba's reason: that Hassan is also his father's son.
Soraya receives self-redemption when she admits her past sins to Amir, who tells her that there is nothing to forgive. "I envied her. Her secret was out. Spoken. Dealt with." But when Amir tries to tell her about his past sins, he is unable to summon the courage to do so. He will have to continue to live with his guilt until he can find away to redeem himself.
Amir finally atones for his past mistakes upon his return to Afghanistan when he locates his nephew, Sohrab, with the intent of returning him to safety. During the beating he takes from Assef, Amir realizes that he has fulfilled his own act of self-redemption.
Are you satifsfied now? he'd hissed. Do you feel better?... My body was broken--just how badly broken I would not find out until later--but I felt healed. Healed at last. (Chapter 22)