The crosses
The seven crosses that the camp commandant constructs are symbolic of Nazi brutality. Traditionally, crosses are used as very positive, oftentimes religious, symbolism. However, in The Seventh Cross, they are the opposite: the Nazis use the crosses to brutally murder people who they don't like. They did so to humiliate them and give a warning to those who may cross them.
Kindness
Kindness is perhaps the most important motif in the novel. Throughout the novel, George is met with kindness from the people he least expects it -- including his former lover, who he abandoned. He is taken in and given shelter by countless people, despite the tremendous risk to themselves, showing the power and importance of showing kindness to people (especially people going through hard times).
Religion
The Seventh Cross parallels the story of God. Much like God, many of the characters in the novel were martyrs, killed because of their religion. And like God, each character -- with the exception of George -- were killed on a cross. The novel is an allegory that warns against the rise of oppressive regimes and which stresses the importance of having freedom of religion in every society on Earth. Without it, the novel shows that people may be killed because of their religion.
The Swastika
The Swastika, which adorns each Nazi uniform and is seemingly omnipresent in Germany, is a symbol not only of the Nazi party, but of hatred. In the early 1930s, Hitler and his Nazi party began their campaign against Jewish people (including Communists, whom Hitler hated because of their beliefs). George was one of those Communists who were sent to concentration camps to their deaths because of hatred.
