That Bach was a master composer is a given. He was also a master recycler, reworking existing pieces to fit a new guise and serve a different purpose. This practice, known as parody, may strike us as unoriginal, but in the Baroque period it was very much an accepted practice. Indeed, for a composer as busy as Bach, it was nothing short of necessity.
The apex of Bach’s parody works is the Christmas Oratorio. It consists of six cantatas, each written for a feast day of the Christmas season. Almost all of the oratorio’s music came from other works by Bach, mostly secular pieces. Without knowing its origins, though, you would be hard pressed to say that the music sounded anything other than perfectly suited to its occasion.