US Catholic bishops received a challenging to-do list Monday as they opened their national assembly — notably to support immigrants and refugees, extend the campaign to curtail clergy sex abuse and work harder to combat gun violence. “The pope has emphasized certain themes: Mercy, closeness to the people… a spirit of hospitality toward migrants, and dialogue with those of other cultures and religions,” Archbishop Christophe Pierre, the papal nuncio, told the bishops as they opened a three-day meeting. Pierre said the bishops should find tangible ways of showing they supported the pope’s merciful message and flexible doctrine, which includes an emphasis on protecting the environment.