The Reformation cantata Ein feste Burg ist unser Gott BWV 80b, published by Carus-Verlag for the first time in a practical performing edition, remained unknown to Bach scholars until well into the 20th century. The evidence of its existence is three fragments of the first folio of Bach's score, which can be dated to the years 1728-1731. By contrast, the long-familiar expanded new version of the cantata with its powerful opening chorus (BWV 80) dates from the 1730s or 1740s. It only survives in a copy of Bach's score, but this allows conclusions to be drawn about the history of the versions; thus, by using the fragments mentioned above, it has been possible to reconstruct the first version. This result is a Bach cantata which is less extensive and in a more concise 'normal' format, with reduced wind instruments. A relaxed, contrapuntal four-part chorale movement with the first and last verses of the Lutheran hymn forms the framework, and the other movements are the same as in the later new version. This simpler cantata is also entirely suitable for celebrating Reformation Day, as Bach did around 1730.