A fierce thunderstorm lashed Memphis, Tennessee, on the evening of April 3, 1968. Weary and feeling ill, the Rev. Dr. Martin Luther King Jr. decided not to give his planned speech for striking sanitation workers at a church hall. He was surprised by an urgent phone call saying a large crowd had braved the weather to hear him. So he went to the hall and spoke for forty minutes, delivering what some say was his greatest speech, “I’ve Been to the Mountaintop.”

The next day, King was killed by an assassin’s bullet, but his speech still inspires oppressed people that they’ll…