This is a slightly unusual cantata in that it's very late: it dates from around 1732-5. We know that on the appropriate Sunday in 1724 (when he first began writing a cantata cycle for St. Thomas' church in Leipzig) that Bach was visiting and giving performances in Cöthen -so it would seem he missed his chance to compose a Sixth-Sunday-after-Trinity cantata then and only caught up with his lapse some ten years later. The cantata is based on a hymn by Paul Speratus, dated 1523, which deals with the highly theological (and rarified!) doctrine of justification by faith alone. That is, good deeds cannot save you, but only faith. The original hymn is long (14 verses!) and the anonymous librettist has condensed it down into 7 movements.
Unusually, too, three of the recitatives are given to the one voice (bass) to sing. Bach generally uses a bass when he wants to imply that Jesus is speaking... so the three recitatives tend to have a feeling of 'this is the word of God' about them: highly didactic and rather intimidating. This, together with a constant harking on about 'the law', makes this cantata highly abstract, quite legalistic -and entirely lacking in opportunities for word-painting or emotional heights or depths. It feels like a lecture your headmaster might give... or, perhaps more appropriate to Bach's particular circumstances, a somewhat tedious sermon the vicar might give. Notice that the opening line of each the bass's arias include the word 'Gesetz': Law. This is no mere accident: in Lutheran theology, God's law (which we cannot help but break) is the thing that will condemn us to judgment. Luther's insight (and relief!) was that salvation by faith alone saves us from the strict consequences of the law being applied to us.
Thus, the bass sets out the existence of God's law in his first recitative, and the consequences of disobeying it. The tenor responds in terror, by pointing out what happens when the law takes its course. The bass replies that, fortunately for us, Christ's sacrifice for us means that faith alone is required; the law no longer strictly condemns us. The soprano and alto aria responds in a bright, enthusiastic re-emphasis that only faith matters. The bass concludes with the summary of the new position: breaking God's law should still prick our conscience, but faith in Christ is the only thing that will save us.
The entire thing is, therefore, a setting out of the central doctrine of Protestantism: salvation by faith alone. Which isn't exactly the sort of thing that is likely to send anyone into an ecstatic frenzy! The music is accordingly 'plain': it's full of subtlety, naturally. This is Bach we're talking about, after all! But it's not full of pathos or heightened emotion, just fairly straightforward doctrinal narrative.
Within this basic 'ground plan', we nevertheless manage to start off proceedings with a rather charming opening chorus. Its use of transverse flute and oboe d'amore duo throughout, supported by simple string interjections, sets a light, spacious tone. The violins' repeated 'rocking' between low and high notes (see bar 2, for an example) contributes to the airiness of the mood. The sopranos carry the chorale tune (and isn't the D natural on their 5th note striking in its effect?), with the other parts supporting in imitation. Listen carefully and you'll notice that the first entry of sopranos+chorus (where the sopranos sing four repeated Bs) is repeated (at bar 59). But at their next entry, they sing B-E-D#-C, and the other voices' accompaniment also changes at this point too. Technically, this means the opening chorus is in AAB form, with the opening two phrases called Stollen and the B-E-D#-C phrase called an Abgesang. Technicalities aside, this structure means your ears know what to expect after a while -and are delighted by the unexpected variation from expectation that arises on the introduction of the Abgesang.
I've already mentioned the anguish of the tenor aria that follows:it is a very angular line and full of chromaticism. This makes it awkward to sing, no doubt: but it also makes for uneasy listening. It doesn't help, either, that as the tenor is singing about being in the depths of the abyss of sin, so Bach has him singing in the very depths of the abyss of his vocal range! Bottom Cs and Ds are difficult notes for a tenor to make sound out properly. You can always trust Bach to match words to appropriate music, though!
Possibly the only other highlight of the work is the soprano/alto duet (movement 5): here the voices and orchestral accompaniment achieve a canonical quintet form. That's to say, the flute and oboe kick off proceedings in canon, with the flute leading. A reverse cannon then has the instruments and voices enter in reverse order. This kind of 'intellectual music' continues throughout: it's clever and artful, of course... but doesn't particularly please these ears.
Summing up: it's an important cantata, in that it tackles head-on the core Lutheran doctrine of justification by faith alone. It would have been important to Bach (a devout Lutheran) -as is perhaps indicated by the fact that he bothered to write it at all some ten years after he'd mostly given up cantata writing. You cannot fail to be hit over the head by its earnestness, though it has a few musical touches which are pleasant enough. Apart from its opening chorus, however, it's unlikely to be a cantata that will lodge in your
mind ear for long, I think.