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When Prime Minister David Cameron unveiled his plan to legalize same-sex marriage last year, Britain’s highest Roman Catholic cleric took to the national pulpit. Cardinal Keith O’Brien decried a “tyranny of tolerance,” calling gay marriage “grotesque” and saying no secular government had the moral authority to legalize such unions. On Monday, O’Brien, one of the church’s most strident voices against homosexuality, abruptly stepped down amid allegations of “intimate” acts with priests. His fall underscored perhaps the greatest challenge for the Roman Catholic hierarchy as it moves to elect a new pope: regaining its own moral authority.