That one thing, King believed, was to commit suicide. Obviously he didn't heed the warning, and even after King's affairs were revealed he continued to be remembered for his civil rights legacy.
The New York Times Magazine on Tuesday published an uncensored copy of the 1964 letter, which Yale historian Beverly Gage stumbled upon in the National Archives. Here's Gage on the letter's origins:
When the Rev. Dr. Martin Luther King Jr. received this letter, nearly 50 years ago, he quietly informed friends that someone wanted him to kill himself — and he thought he knew who that someone was. Despite its half-baked prose, self-conscious amateurism and other attempts at misdirection, King was certain the letter had come from the F.B.I. Its infamous director, J. Edgar Hoover, made no secret of his desire to see King discredited. A little more than a decade later, the Senate's Church Committee on intelligence overreach confirmed King's suspicion.