Elie Wiesel's relationship with God in Night is a fraught and difficult one. Though he believes in God steadfastly in the beginning, that belief begins to deteriorate until he not only lacks belief in the goodness of God but is also angry at the very idea of God.

Wiesel was a deeply religious man who believed in God and lived to reflect that belief. However, his faith is tested and all but destroyed during the Holocaust. His mother and sister are gassed to death. He and his father endure forced labor in the concentration camps. During his time there, he saw many terrible things happen to innocent people; these people had faith in God and it did not save them, which begins to erode Wiesel's belief.

People in the book ask where God is and why he doesn't intervene. At one point, looking at the corpse of a hanged child, he says that God is there, in the corpse, killed and dead.

Wiesel writes:

In days gone by, Rosh Hashanah had dominated my life. I knew that my sins grieved the Almighty and so I pleaded for forgiveness. In those days, I fully believed that the salvation of the world depended on every one of my deeds, on every one of my prayers.

But now, I no longer pleaded for anything. I was no longer able to lament. On the contrary, I felt very strong. I was the accuser, God the accused. My eyes had opened and I was alone, terribly alone in a world without God, without man. Without love or mercy. I was nothing but ashes now, but I felt myself to be stronger than this Almighty to whom my life had been bound for so long. In the midst of these men assembled for prayer, I felt like an observer, a stranger.

Despite this anger, the last mention of God in the book is an unspoken prayer to "this God in whom I no longer believed" to never abandon his father the way Rabbi Eliahu's son did.

One thing to note, however, is that in the Nobel Prize acceptance speech posted as an afterward, Wiesel says that he still has faith. He says,

But I have faith. Faith in the God of Abraham, Isaac, and Jacob, and even in His creation. Without it no action would be possible.

Even though his experiences tested his faith, eventually he grew back into that faith. Since the book ends abruptly with him being freed from Buchenwald, it doesn't show how his faith grew after he left the camp.

In Elie Wiesel's Night, he struggles with his faith in God as his situation worsens.
 
Towards the beginning of the memoir, Wiesel's relationship with God is strong. He talks about how he studied the Talmud during the day, attended the synagogue at night, and even wanted his father to find him a master who could help him study Kabbalah. All of this, though, describes his life before he and his family are taken to Auschwitz.
 
After spending his first night in the concentration camp and seeing so much evil, such as babies being killed, Wiesel's faith is shaken to say the least. In his own words, he says, "Never shall I forget those moments that murdered my God and my soul and turned my dreams to ashes" (Wiesel, 34). 
 
Soon, Wiesel's faith begins to decline rapidly. While other Jews talk about the "mysterious ways" of God and the sins of their people, Wiesel simply stops praying. He says, "I was not denying His existence, but I doubted His absolute justice" (Wiesel, 45). Then, Wiesel witnesses the hanging of three prisoners, one of whom is a child, and questions why God lets these things happen to people.
 
By page 68, Wiesel seems almost sure that God has abandoned them. While he still prays with the other prisoners, his heart is no longer in it. Regarding this, he says, "My eyes had opened and I was alone, terribly alone in a world without God, without man" (Wiesel, 68). Though he never outright denies God's existence, his doubts begin to overwhelm him. 
 
Still, though, he continues to reference, and pray to, God. After Rabbi Eliahu's son purposely leaves him behind during a march, Wiesel prays, "'Oh God, Master of the Universe, give me the strength never to do what Rabbi Eliahu's son has done'" (Wiesel, 91). That said, though he's struggling with his faith, it's evident that some part of him still believes. 
 
By the end of the memoir, Wiesel and his father are transferred to Buchenwald. His father gets sick and dies, yet he cannot even cry. Not too long after, the American military shows up and frees them all. 
 
In his Nobel Peace Prize Acceptance Speech, given in 1986, Wiesel gives thanks to God, which proves that, somehow, he was able to retain his faith after the horrifying years he spent in Auschwitz and Buchenwald. He opens his speech saying, "Words of gratitude. First to our common Creator" (Wiesel, 117). He then goes on to recount his experience.
 
Eventually, Wiesel brings up the conflict between Israel and Palestine and urges the people of the world to make the violence stop. He says, "Should Israel lose but one war, it would mean her end and ours as well. But I have faith. Faith in the God of Abraham, Isaac, and Jacob, and even in His creation. Without it no action would be possible" (Wiesel, 120).