What was the flaw in Shepsel's strategy?

Misha is living what seems almost a carefree existence as a child thief working the streets of Warsaw. One day, he meets a Jewish girl called Janina. As the Nazis tighten their grip on Poland, life becomes harder and more dangerous for the country's substantial Jewish population. Eventually, all of Warsaw's Jewish families are herded into a ghetto, where they suffer chronic food shortages, disease, and massive over-crowding. It's a desperate situation for Janina's family, and desperate situations call for desperate measures. Janina's Uncle Shepsel comes up with what he thinks is a way to escape the ghetto: he's going to recant his Judaism and declare himself a Lutheran instead.
Unfortunately for Shepsel, his strategy for survival is doomed to failure. The Nazis don't regard Jews as forming a religion, but a race. As such, they believe that once a Jew, always a Jew. It's too late, then, for Shepsel to do anything to change either his identity or his fate.