It's difficult to create a thesis statement without first choosing a topic. Once you choose a topic, thing about something about that topic that you'd like to prove. For example, Night has several examples in which the Jews are put in such terrible situations that they begin to act like animals, forgetting their own families in order to survive. Your thesis statement could be something like this:
When placed in extreme survival situations, Eli and other Jews in his situation behave in animalistic ways.
Another main idea throughout the text is the way in which Eli's faith is tested throughout the difficulties he endures. He finds it difficult to trust that God is a benevolent being, or even exists at all, in the face of such horror. You would trace this progression throughout the text, and your thesis statement for this essay could be:
Because of severe trial and adversity, Eli Wiesel questions his faith in God, despite being a faithful young man prior to his experiences.