It was 45 minutes into his lecture when the rabbi pulled out an AR-15. “Who thinks, by show of hands, that we should be carrying more guns in shul?” Rabbi Raziel Cohen asked the crowd at a Westside Chabad synagogue Wednesday night, during an active-shooter seminar organized in the wake of the deadly attack at Chabad of Poway. Half the room raised their hands. In the days since the shooting, Chabad leaders in California have scrambled to secure public safety grants and to calm frightened congregants, mobilizing hundreds more through active-shooter drills and community defense training. In Southern California, religious...