PROVIDENCE, R.I. – A plan to open what would be the nation's only museum centered on the trans-Atlantic slave trade would focus on the Episcopal Church's role in its history and the sometimes-buried legacy of slavery in northern states like Rhode Island. The museum at the shuttered Cathedral of St. John, a church where slaves once worshipped, would explore how the church benefited from the trade and helped bring it to an end, said Bishop Nicholas Knisely of the Diocese of Rhode Island.