The Reformation Festival is one of the major annual events in the Lutheran calendar; it is celebrated on October 31, the day on which Martin Luther nailed his 95 theses to the door of the castle church at Wittenburg in 1517. BWV 80 ("A mighty fortress is our God"), one of Bach's most famous cantatas, is one of two that Bach composed for the Festival. Unlike BWV 79, its companion, BWV 80 has a highly complex history. It appears to have started life as a Lenten cantata (BWV 80a) composed during Bach's period in Weimar, probably in 1715; in 1723, his first year in Leipzig, Bach expanded the work for use at the Festival that year, in the process dropping the great chorus based on Luther's famous hymn in favor of a simple four-part harmonization. At some later date Bach again revised the cantata, replacing the opening chorale with a chorus in the old polyphonic style. Confusion over the instrumentation of BWV 80 is exacerbated by the loss of Bach's original score and parts; the version known today comes from a copy made by Bach's pupil and son-in-law, Johann Christoph Altnickol. Further confusion resulted when trumpet and timpani parts composed by Bach's eldest son, Wilhelm Friedemann Bach, found their way into the first printing of the work; although spurious, they are still included in many of the performances heard today.
Luther's well-known hymn dominates the work; aside from its prominence in choral movements, it serves as a cantus firmus in a duet for soprano and bass (No. 2). A second soprano aria with only continuo accompaniment follows the succeeding bass recitative; its small scale and reflective character provide relief from the prevailing grandeur of the work. The other solo number is a duet for alto and tenor (No. 7) with an obbligato part for oboe da caccia (an oboe in the original Weimar version). The concluding chorale returns to Luther's hymn, setting the final strophe as a simple four-part harmonization.