In 1723 Bach was appointed Kantor at St. Thomas’s Church, Leipzig. For the next six years he brought his small stock of church cantatas up to about 300, (which was five more or less complete sets of cantatas for the Church Year). This provided for all his needs for his work at St. Thomas’s for the rest of his time there.When called upon to produce new music, he was in the habit of taking movements from earlier works and adapting them, reworking them where necessary. Some movements underwent several re-workings. The term parody signifies music which has been recycled in this manner.
Some of Bach’s greatest music has a substantial parody element. An example is the Mass in B minor. At the accession of the new Elector, Bach presented him with the Kyrie & Gloria of the Mass, (Kyrie and Gloria being the parts of the Lutheran Mass set to music). Then in 1748-49 he added the Credo, Sanctus, Benedictus and Agnus Dei, completing what we now know as the B minor Mass. His reason for doing this is not clear*, but there was no new music: it is in effect an anthology of some of the finest bits of his earlier cantatas, the words being replaced by those of the ordinary of the mass. Some of his adaptations are quite radical (such as the wonderfully moving end of the Crucifixus, adapted from the cantata Weinen, Klagen, Sorgen, Zagen).
* It is thought by some that it was in support of a job application for Dresden.