How does Liesel Meminger truly become the book thief?

Liesel brings Rudy to the mayor's house with the intention of stealing The Whistler, though Rudy things they are trying to get food. Liesel climbs through an open window and gets the book; they race off at first but Rudy loses her shoes and has to go back and retrieve them. When they get to their homes, Rudy for the first time calls Liesel "book thief."