One hundred and twenty people packed the Chabad. As they trudged in, stamping snow from their boots and unwinding woolen scarves, I realized I wouldn’t find a bigger gathering of Jewish people anywhere in my Wisconsin town. And there I was, a Christian woman at the keyboard, with a group of musicians playing Jewish songs. The violinists, seated so close to me that I could have plucked their strings, cradled instruments that were almost a century old. The violins were once owned and played by Jewish people during the Holocaust. Rescued as part of the Violins of Hope project, they...