Early one morning last fall, Nathaniel Felder left his home, in Maplewood, New Jersey, dressed in a navy sweatshirt and a gray yarmulke. After putting on a backpack, he walked to Irvington Avenue, the location of Congregation Beth El, a Conservative synagogue in neighboring South Orange. Outside the shul’s entrance, a sign greeted visitors: “We Stand with Israel and We Pray for Peace.” Felder reached into his backpack and unfurled a sign of his own. It proclaimed, “Starvation Is Against Jewish Values: Our Support of Israel Cannot Be Unconditional.” Felder angled his sign so that it would be visible to...