When Amir is in fifth grade, he comes home from school after a lesson from his mullah on sin. He has just learned that drinking alcohol is a sin, and he discusses this with Baba, who is pouring himself a whiskey.  Baba expresses his opinion of mullahs, saying, "You'll never learn anything of value from those bearded idiots" (17). He then goes on to explain to Amir his own thoughts on sin. He tells Amir there is only one sin, which is theft.  All sins are "a variation of theft" (17). He goes on to expand on this with examples for Amir. If a person is murdered, the murderer has stolen a life.  A lie is the theft of a truth, and cheating is a theft of "the right to fairness" (18).  Amir absorbs this lesson, and Baba gets more whiskey. As we look at this scene within the context of the entire plot, it is easy to see why Baba feels this way, but we can also see that Baba is somewhat hypocritical and that Amir is to some degree damaged by this discussion. 

We know that Baba's own father's life was stolen by a murderer, which is likely to have formed to a large degree his take on what constitutes a sin.  However, we also learn that Baba has been guilty of the sin of theft, too, in "taking" Ali's wife from him.  That he feels guilty about this is implicit in his treatment of Ali and of Hassan, who is the product of this taking. He loves Hassan, but I think there is also some guilt. Amir's response to this "lesson" on sin is to feel his own pain and guilt for having taken his own mother's life during childbirth, a theft of her life and the theft of his father's wife.