In 1733, Bach found himself in a competition with Vivaldi, Albinoni, Telemann, and several other musicians for a position with the court orchestra in Dresden -- Bach was always looking to improve his job status, and he sent along some portions of a Mass to demonstrate his skills as a composer.
Well, Bach didn't get the job. Neither did Vivaldi, Albinoni, or Telemann. But eventually, Bach completed his Mass in B minor, and we turned out to be the true winners of the 1733 competition. We'll hear it this Sunday. (Nov.11).