Recently I saw a riveting new play, “My Report to the World: The Story of Jan Karski.” In 1943 Karski, a member of the Polish Catholic underground, was the first eyewitness to the Warsaw ghetto and a Nazi concentration camp to reach the U.S. He sought to convince President Franklin D. Roosevelt and Supreme Court Justice Felix Frankfurter that Judaism was being systematically wiped out by the Nazis. Frankfurter told Karski he couldn’t believe it because the horror was unfathomable.

A similar story is playing out again, as Christians...