For three centuries, religious difference was the defining issue of our national life. From roughly the Act of Supremacy in 1534 to the Catholic Emancipation Act of 1829, religion determined our conflicts abroad and tore us apart at home. In that period, Roman Catholics were cast as the religious outsider. Now, at the end of two centuries of mostly peaceful accommodation and compromise, it seems certain that Andy Burnham will be the first meaningfully Roman Catholic Prime Minister in British history. Yet Burnham will have to address freshly prominent and volatile challenges of religious difference. We are entering a new era...