Bach's Cantata 80 is an elaboration of Luther's “Ein Feste Burg.” The second movement of the Cantata is a duet of soprano and bass, the former singing the second verse of Luther's hymn while the bass sings an embellishment promising the victory of God.
The bass part begins and ends with lines from librettist Salomon Franck: “Alles, was von Gott geboren / Ist zum Siegen auserkoren” (All who are born of God / are chosen for victory). The lines are appropriate accompaniment to the hymn's reminder that “our striving would be losing” if we “in our own strength confide.” Christ Jesus, Lord of Armies, wins the battle.
Between the repeated couplet are three lines about baptism: “Wer bei Christi Blutpanier / In der Taufe Treu geschworen / Siegt im Christo fur und fur” (Whoever by Christ's blood-banner / Is sworn into the fealty of baptism / Wins with Christ forever and ever).
The juxtaposition of the texts brings out a hidden sacramental motif in the hymn. Christ leads his people to victory, carrying his blood-banner, the cross, and the company that follows has been enlisted into his regiment by baptism. The baptized join the hosts of the Lord of hosts. Our God is a mighty fortress to those who have passed through the sea, baptized into the new Moses.
Whether or not Luther had any such thing in mind in the hymn, Franck's gloss is consistent with Luther's theology and theologically sound. Next time you sing “Ein feste Burg,” sing it as a hymn memorializing your baptism.