[cynicalkane]

To refer to chorale melodies as "popular music" is really a stretch. For instance, Wikipedia tells us that "O Haupt voll Blut und Wunden" was in use as a chorale by 1656. You'd be really stretching to call this and other chorales a "popular input". I've never read or heard anything to indicate that church melodies were significantly popular in the 1700s.

Popular music was stuff like song, opera, maybe some easy instrumental stuff (though I can't remember when music publishing took off). Bach wrote few songs, no opera, and his instrumental output was very difficult and went largely unpublished.

Also, I don't know what the 'Christmas Cantata' is. Google wasn't any help in clarifying it. Bach wrote about two billion settings of that chorale, so that's not much help.