How is it that Amir began to pray again after so many years away from it?

In Chapter 25, Amir is at the hospital with Sohrab, who has attempted to taked his own life. While waiting, Amir finds a bedsheet and kneels on it to pray for the first time in over fifteen years. He mumbles the phrases of prayers he still remembers, his belief in God suddenly renewed. In his prayers, he begs God to let Sohrab live; he will do anything to ensure the boy's safety, saying, "My hands are stained with Hassan's blood; I pray God doesn't let them get stained with the blood of this boy too." He recounts what he saw when he opened the bathroom door; Sohrab lay dying in the bloody bathwater, having slit his wrists with Amir's razor. Finally, a doctor tells Amir that Sohrab will live.