Trace Elie Wiesel's faith. Does he ever regain it?

It is debatable whether Eliezer completely loses his faith in Night. However, there can be no doubt that his beliefs undergo substantial change throughout as a result of his experiences. Initially, Eliezer's faith is instinctive, unthinking, and an integral part of his cultural heritage. When a fellow prisoner asks him why he always cries when he prays, Eliezer is unable to give a satisfactory explanation. The same thing occurs when the man asks him the follow-up question "Why do you pray?":

Why did I pray? Strange question. Why did I live? Why did I breathe? "I don't know," I told him, even more troubled and ill at ease. "I don't know."

We sense that Eliezer has never really reflected on his faith before. However, the horrors of life in Auschwitz have changed all that. They challenge his faith; they force him to question everything he has ever believed in. Under the daily torments of life in the camp, Eliezer's understanding of what God is changes significantly. At first, he does not outright reject God's existence. Instead, he refuses to believe that he is a just, merciful God. He cannot reconcile such a God with the sheer evil and moral degradation he is forced to witness each day:

Blessed be God’s name? Why, but why would I bless Him? Every fiber in me rebelled. Because He caused thousands of children to burn in His mass graves? Because He kept six crematoria working day and night, including Sabbath and the Holy Days? Because in His great might, He had created Auschwitz, Birkenau, Buna, and so many other factories of death? How could I say to Him: Blessed be Thou, Almighty, Master of the Universe, who chose us among all nations to be tortured day and night, to watch as our fathers, our mothers, our brothers end up in the furnaces? Praised be Thy Holy Name, for having chosen us to be slaughtered on Thine altar?

It is only during the hanging of the young boy, the Kapo's assistant, that Eliezer's faith finally appears to vanish altogether. The young boy seems so fresh-faced and so innocent. His slow, painful death at the end of a rope represents a new low—even by the standards of Nazi brutality. As the prisoners are forced to watch the sordid spectacle, a voice cries out "Where is God now?" In his mind, Eliezer now believes that God has effectively died with the young boy.
Yet later on, Eliezer appears to have some belief in the existence of God once more, albeit in a somewhat ambiguous sense. However, this time he is no longer in the position of a suppliant; he is an accuser, castigating God for what he sees as his indifference toward the suffering of his chosen people:

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.

Whether or not this represents Eliezer's final position on the matter, we cannot know for sure. However, there seems little doubt that he derives new strength from his apostasy. Elie feels that, even if God does exist, he is no longer worthy of Eliezer's prayers; he is worthy only of ceaseless condemnation.