It's hard to provide page numbers as there are many different copies of the book, but here are some quotes and examples.
Throughout the novel, Elie tries desperately to remain faithful to God. He clearly has a deep desire to love/worship God, especially when we see him tutored by Moshe the Beadle. However, during the darker moments of the novel, we see him quickly questioning his faith. After all, if God is so faithful and just, why would He turn his back on His chosen people?
Examine this quote: “Never shall I forget that nocturnal silence which deprived me, for all eternity, of the desire to live.”
Yes, Elie is referring to the silence at night in the camps. However, he is also making a reference to the silence that existed when God was not close--without God, his existence felt like one eternal night (thus the name of the book). This quote is the strongest example of his faith faltering.
Another example is of the boy playing the violin when they have run all night from one concentration camp to the next. This scene is eerie, and Elie says, "It was pitch dark. I could ehar only the violin, and it was as though Juliek's soul were the bow. He was playing his life. The whole of his life was gliding on the strings--his last hopes, his charred past, his extinguished future. He played as he would never play again...when I awoke, in the daylight, I could see Juliek, opposite me, slumped over, dead. Near him lay his violin, smashed, trampled, a strange overwhelming little corpse." This is another moment where Elie questions God. How could a God make something so small and beautiful (both Juliek and the violin) and let it be completely destroyed??
Finally, there is the relationship between the prisoners and the Nazis. Elie at first believes that they are comforting and going to help them, but eventually one of the Kapos says to him, "Here, every man has to fight for himself and not think of anyone else. . . . Here, there are no fathers, no brothers, no friends. Everyone lives and dies for himself alone". At first we see no proof that Eli takes this to heart, however, he later begins to refer to the Kapos as functionaries of death and we see him abandon his father on occasion. Elie is truly changed by the death camps--he doesn't know how to hold on to his God and His promises.