This is a very interesting question because this powerful novel shows us the change in faith of the main character, Eliezer, who begins the story as a fervent Jew, but through his experiences, loses his faith in a God that could allow such immense and unjust suffering. One key experience you will no doubt want to focus on is the hanging of a young boy for his involvement in the resistance. Note how he is described:
This one had a delicate and a beautiful face - an incredible sight in the camp.
Just before the boy and his two compatriots are hung, someone behind Eliezer is heard to say "Merciful God, where is He?" As Eliezer and all the other prisoners are forced to walk past the victims, note how the three hung men are described:
The two men were no longer alive. Their tongues were hanging out, swollen and bluish. But the third rope was still moving: the child, too light, was still breathing...
The child, because of his lack of weight, suffered a far crueller death, as he is left to choke to death slowly and excruciatingly. Eliezer reports that it took him more than half an hour to die, whilst all the time Eliezer and the other prisoners were forced to watch him die. As the same voice again repeats his question, Eliezer hears within himself an answer to where God is:
"Where is he? This is where - hanging here from this gallows..."
This, then, has to be the defining point in the novel when Eliezer irrevocably lost his faith in God, and saw him as dead, strung up like the angelic boy he was forced to watch die.