The solo in this week’s service is an aria from Bach’s cantata 80, Ein feste Burg ist unser Gott. This cantata has a complex history, being an adaptation of an earlier cantata, Alles, was von Gott geboren, which Bach wrote in Weimar probably in 1715 or 1716 for performance on the third Sunday of Lent. The original cantata began with a paraphrase of a verse from the first epistle of John, “For whatsoever is born of God overcometh the world” (1 John 5:4) and ended with the second verse of Martin Luther’s chorale, “A mighty fortress is our God.” When Bach moved to Leipzig, where cantatas were not permitted in the service during Lent, he adapted the work for use on Reformation Day (October 31) by including all of the verses from Luther’s chorale, adding a magnificent new opening movement to accommodate this. The aria, “Komm in mein Herzenshaus,” was reused in the later cantata apparently unchanged (the music for the Weimar cantata is lost), and is intimately scored for soprano and continuo (cello and keyboard instrument). The text, “Come in to my heart’s house,” is underlined by an elegant melody which floats downwards, perhaps suggesting the descent of Christ from heaven.