Cantata no. 80: Ein' feste Burg ist unser Gott, BWV.80

Ein feste Burg ist unser Gott (A Mighty Fortress Is Our God), BWV 80 (also: BWV 80.3), is a chorale cantata for Reformation Day by Johann Sebastian Bach. He reworked it from one of his Weimar cantatas, Alles, was von Gott geboren, BWV 80a (also: BWV 80.1). The first Leipzig version of the church cantata, BWV 80b (also: BWV 80.2), may have been composed as early as 1723, some five months after Bach had moved to Leipzig. Some years later he reworked the cantata one more time, writing an extended chorale fantasia as its opening movement. The text of the BWV 80a version was written by Salomon Franck and contained one stanza of Martin Luther's hymn "Ein feste Burg ist unser Gott"; for his chorale cantata versions, BWV 80b and 80, Bach added the complete text of this Lutheran hymn.

Bach scored the cantata for four vocal soloists, a four-part choir and a Baroque chamber ensemble of up to three oboes of different kinds, strings and continuo. After his death, his son Wilhelm Friedemann Bach arranged the first and fifth movements, adopting a new text and adding trumpets and timpani.