Ein feste Burg ist unser Gott, BWV 80, 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. The first Leipzig version of the church cantata, BWV 80b, 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.
The above text from the Wikipedia article "Ein' feste Burg ist unser Gott (Bach cantata)" text is available under CC BY-SA 3.0.