This is an enormous question, and we must keep in mind that when we ask it regarding Elie in Night, we are not approaching it as character development as we would for analysis of fiction. On the contrary, we are considering the changes a man--indeed, a boy--went through in an horrific and very real series of events.
In the beginning of his life, Elie was devoted to the Orthodox Jewish religion. He followed regular prayers and practices, then at night even studied the mystical Jewish secrets called Kabbalah. Then, as he went from camp to camp and saw atrocity after atrocity, death after death--a child with God's image in his eyes hung, with God hung beside him--he felt God die in his heart and ceased to believe in the God of the Jews, this even though he still uttered prayers of desperation to the God he no longer believed.
Elie and his father have a relationship built upon paternal and filial love, though Mr. Wiesel is not affectionately demonstrative. Elie feels devoted respect for him, and his father feels deep love and admiration for Elie. At the camps they cling to each other to stay together. They help each other to get into the right lines and rooms so as to keep their chances of living as high as possible. Elie watches over his father who seems to continually grow older and ever weaker. When Mr. Wiesel is finally broken by work and starvation, Elie gets him doctors, takes care of his dysentery the best he knows how to, and risks staying by him in their bunks. In the end, when Elie awoke on January 28th of 1945 to find his father had been taken during the night to the crematorium, his soul could not weep for the father it had loved but felt only relief:
if I could have searched the recesses of my feeble conscience, I might have found something like: Free at last!
Elie was a boy when the horrors of his night started. He believed he lived in a world ordered by light that limited evil and harm. When he left at the end of the time of the camps, he knew he lived in a world where "absolute evil" was possible. When he left the camps, he saw in his reflection the eyes of a corpse. The Yiddish original of Night contains a passage that illuminates the change in his view of the world:
We believed in God, trusted in man, and lived with the illusion that everyone of us has been entrusted with a sacred spark from the Shekhinah's flame; that every one of us carries in his eyes and in his soul a reflection of God's image.
To answer this question, first re-read the very beginning of Night. Pay attention to how focused Elie is on his religious studies, to how he describes his father's position of importance in their community, and to his hopes for his own future. Even when they are in the early days of their imprisonment, Elie holds onto those views.
Then go back through the part of the book which leads up to his father's death. Notice how he is questioning his religious upbringing and the existence of God; he finds himself in the "father" or leadership position, directing his father and pulling him along through their experiences; he no longer seems to think about a future, not even much in terms of surviving the present. Initially, he cared about helping others; he has lost that value, too, after his experiences.
You might want to use a book summary or other assignments and notes that you have completed while you read Night to help you find just the right passages to clarify your answer and provide appropriate support for it.