This one kind of goes without saying, don't you think? The Holocaust was all about prejudice. The Nazis judged and persecuted people based on their race and religion. But while the Nazis singled out Jews because of their differences, the Johansens and the Rosens are friends in spite of their differences. The connection between these two families in Number the Stars reminds us just how pointless (and baseless) prejudice really is.
In Number the Stars, religious difference seems so normal for so many of the characters. This emphasizes the absurdity of the Nazis' desire to punish people because of their religion.
Annemarie and her family are not in any danger from the Nazis until they decide to help the Rosens. In a way, they choose to share in their friends' persecution and danger.