WARSAW, Poland—As Nazi troops imposed their terror on Warsaw, an 18-year-old Polish girl slipped into a Warsaw church with an elderly rabbi to teach him how to dip his hand in holy water and cross himself. The rabbi, his newly shaven beard leaving his cheeks white, approached the lesson with gravity, skimming the water in the church font and crossing himself with slow reverence, hoping this would help him pass as Catholic. “You’ve already exposed yourself! You’re dead already!” the teenager whispered in his ear, and showed him how to perform the gestures the way Catholics did.