While sifting through material at the Virginia Theological Seminary in Alexandria earlier this year, Nick Gentry made a once-in-a-lifetime discovery. There, among the collection belonging to John Melville Burgess, the first African American to lead an Episcopal diocese, was an original draft of the Rev. Martin Luther King Jr.’s seminal “Letter From Birmingham Jail.” Gentry, who served an internship at the seminary, said he thought he’d found a c…