JERUSALEM—In Jerusalem’s Old City, a group of Israeli teenagers recently banged on the door of the headquarters of the Armenian Apostolic Church and asked if there were Christians inside it. They wanted to spit at them.The boys said it was a religious duty, or mitzvah, to humiliate Christians, or, as they called them, idol worshipers. “We are supposed to hate them,” explained one teen, wrapped in an Israeli flag. They were among the thousands of mostly young Israelis who on that May afternoon marched through the historic Old City for a national celebration that marks the capture of East Jerusalem...