Ein feste Burg ist unser Gott
The cantata opens with a chorale fantasia "with contrapuntal devices of awe-inspiring complexity". The movement in D major and common time elaborates on the first stanza of the hymn "" ("A Mighty Fortress Is Our God"). It adopts a motet technique of having the instrumental and vocal lines follow each other closely. The Bach scholar Klaus Hofmann noted that the style relates to the "vocal polyphony of the sixteenth century", when the Luther hymn was written. Structurally, the movement repeats the first two phrases, adds four new shorter phrases, then concludes with another iteration of the second phrase, all performed on oboe. All four voices "discuss each phrase imitatively as a prelude to its instrumental entry", using fugal devices. The chorale also makes use of the cantus firmus, which is exclusively played in the basso continuo and oboe lines (in the movement's original form).