The “moment of truth” is what the Spanish call that instant when a bullfighter chooses to make the final thrust of his sword.
The phrase was introduced into English in Ernest Hemingway’s 1932 novel Death In The Afternoon.
The timing of that final move by the bullfighter is critical for both the matador and the animal, and so el momento de la verdad, or the moment of truth, became synonymous with any critical decision.
