Bach’s solo cantatas are, in some sense, his only true cantatas since the term `cantata’ was drawn from an Italian genre which was designed for solo voice. The solo cantatas offer a more contemplative approach to the text since the standard form for a single soloist was the aria, the form that so poignantly and passionately expresses, and internalizes emotion.
Cantata BWV 55, Ich armer Mensch, ich Sündenknecht, belongs to a series of solo cantatas that were composed in 1726. The solo tenor in this cantata personifies the Parable of the Unmerciful Servant as described in Matthew 18: 23-35. The over-arching formal structure of the cantata devotes the first two movements to reflections on the sinfulness of man, with the remaining three movements focusing on the mercy of God.
The opening aria and secco recitative that follows express the antithesis of the merciful Lord and the hard-hearted servant. Of special interest is the role the woodwinds play, coupled with the absence of the viola in this initial aria. The flute and oboe d’amore frequently move in parallel thirds and sixths with both violins. These instruments, together with the continuo, create a five-part musical structure that opens up to six parts upon the entry of the tenor. By leaving the violas out of the texture and writing with a preference for the high range of the tenor, Bach creates the effect of a sinner writhing in pain as he attempts in vain to rid himself of the great burden that he is carrying. In the first recitative, the unknown librettist uses some of Psalm 139: 7-10 to portray the omnipresent God. He cannot escape the 'switch of sin' and the presence of his Maker, no matter how high and tortuously he sings.
In the third and fourth movements, the subject now turns from sinful humankind to divine mercy. The second aria, "Erbarme dich", with an obbligato flute, is filled with the anguish expressed in the first aria. The pleading of the sinner is presented by means of interval leaps of a sixth and falling seconds, but also by virtuoso passages on the flute. The rising minor-sixth, which was banned from the strict polyphonic idiom of the Renaissance, is exploited here for special effect as a means of conveying 'exclamation.' In the B section, the second statement of the words "deinen Zorn…stillen" (still your anger) elicits a sudden silence, making all the more dramatic the following repetition of "Erbarme dich."
It is interesting to note that both the third and fourth movements begin with the exclamation "Erbarme dich" (Have mercy), the opening words of the great alto aria from the St. Matthew Passion. The latter was written only a few months later, so it is likely that this cantata influenced Bach’s later ideas. The final chorale (Bach sets the same verse in the St. Matthew Passion) is a simply harmonized, comforting expression of God’s mercy.