Whose death symbolizes the loss of Elies faith in the "Night Book"?

It's not Juliek. It's the hanged Pipel. Elie hears someone ask where God is, and he responds with "Where is he? He is hanging here on the gallows." (paraphrased.) What Elie is saying is that God is dead. He is stating that God has given up on them. Later, Elie refuses to take part in Jewish rituals because of this and clearly states many times that he is only staying alive and fighting because of his father: never does he mention his faith as a reason for going on. After the Pipel dies, Elie becomes rooted in human values and tactile thoughts; he abandons his spirituality because he has lost his faith.