In the months after his father’s murder in early 1999, those months stretching formlessly between the mourning ritual of shiva and the impending trial of a suspect, Rabbi Joel Mosbacher received many messages of solace. There was one type, however, that tested every atom of clerical forbearance he possessed. “People said in this trying-to-be-helpful way, ‘This will make you a better rabbi,’ ” Rabbi Mosbacher, 43, recalled. “And nothing made me angrier. I didn’t want to be a better rabbi. I wanted my dad back.” ...While the rabbi had done a bit of tentative speaking on gun control as early as 2000, his commitment accelerated after 2006, when he first learned about the Industrial Areas Foundation. As a legacy of the renowned community organizer Saul Alinsky, the foundation largely mobilizes religious congregations. Its style, far from being earnest, is proudly combative.