Martin Pearlman is the founder, music director, and conductor of Boston Baroque, the orchestral ensemble and chorus that performs repertoire from Monteverdi to Beethoven.
Recycling one's own music was a common practice in Handel's day, and throughout the Baroque. Interestingly, or strangely, one of the best known choruses from Handel's Messiah was originally a torrid love song.
Here's another surprise Messiah fact from conductor Martin Pearlman: "The most consistent time that he performed it tended to be in May. It was not related to holidays. This is not a church piece, it's not meant for religious observance necessarily. The person who took the biblical texts and assembled them called it a 'fine entertainment,' which is a very interesting way of thinking about the work. So people have used it for Christmas, they've used it for Easter, and Handel used it for neither one."