How could he have lied to me all those years? To Hassan? He had sat me on his lap when I was little, looked me straight in the eyes, and said, There is only one sin. And that is theft... When you tell a lie, you steal someone’s right to the truth. Hadn’t he said those words to me? And now, fifteen years after I’d buried him, I was learning that Baba had been a thief. And a thief of the worst kind, because the things he’d stolen had been sacred: from me the right to know I had a brother, from Hassan his identity, and from Ali his honor. (Hosseini, The Kite Runner, Google Books)
Because of this essential lie, Baba's relationship with Amir was always strained; even though Baba's lessons stayed with Amir for his entire life, and even though he found his father admirable in many ways, the revelation of a lie held throughout his entire life taints all of his memories and shows just how much and how deeply Baba hid -- both from Amir, and from himself.