Elie's book teaches so many lessons about what it means to be human, but arguably, the most important lesson it teaches is that it is hard to be human and humane. The very fact that Nazi Germany could seek to eradicate a group of people is proof that something is wrong in our world. The extent of evil that the book portrays is absolutely astounding; it is almost unbelievable. Here is one quote:
He seemed to be telling the truth. Not far from us, flames, huge flames, were rising from a ditch. Something was being burned there. A truck drew close and unloaded its hold: small children. Babies! Yes, I did see this, with my own e y e s ... children thrown into the flames. (Is it any wonder that ever since then, sleep tends to elude me?)
Moreover, as Elie, his family, and other Jews face these horrors, they find it hard to support each other. Family members begin to betray each other. The point is that it is difficult to be humane.
Second, the book teaches us that showing compassion is part of being human. For example, the Jewish girl who was kind to Elie is a perfect example of compassion. She risked her own life to say a kind word to Elie and to give him some bread when he was at a low point. As hard as it might be, the book teaches us to be human is to be kind and compassionate to one another. This takes courage and commitment.
As for justice, the book does not speak about what a just society should look like, but we can deduce that a just society should be a society that seeks the welfare and wellbeing of all people, no matter how different they might be.