HAIFA, Israel — St. Louis the King Cathedral is festooned with string lights for the annual Christmas tree-lighting ceremony. The multitude pressed between the alabaster-like walls of the churchyard is so dense, it almost feels like every one of Haifa's estimated 4,000 Maronite Catholics is here. Before the lighting ceremony and the fireworks, the Rev. Yousef Yacoub invites a rabbi onstage. Yacoub stands beside Na'ama Dafni of Or Hadash, a Reform congregation, as she lights a blue-and-white braided candle and says a nondenominational prayer. "It is a great honor and privilege to be with you today, to kindle lights of...