An illuminating passage from Guenther Stiller's Johann Sebastian Bach and Liturgical Life in Leipzig. Emphases mine.
Music once meant something very different to the Church. Instead of being a theatrical element of a service, it was once regarded as something much higher. The quote below illustrates what we've lost.
I could hardly ask for a better summary of my own perspectives on this.
[Bach's Music in Leipzig] ... becomes fully intelligible only when we bring the texts used by Johann Sebastian Bach and the theology they embrace into our purview, as in general Bach's cantata music, dependent on a text as it is, can be understood only as an organic unit of art. This fact must simply be a fundamental presupposition for every theological consideration concerning the essence of the Bach cantata.
For this reason a frequent approach of the past that distinguished between form and content is completely illegitimate, for the distinction between form and content is useful for the music of the 19th century but does not apply to the problems of older music. Actually, the form of the music itself is always an element of content already!
[The text used by Bach and others] ... was intent on a compositional technique that could do justice to language and interpret the Word successfully. Already its close affinity to rhetoric shows this, so that it is not surprising that the Sermon and the musical exegesis of Scripture on the evangelical scene come together in the 'sermon music' [Predigtmusik].
Luther...wanted to see music employed exclusively "in the service of exegesis and the enlivening of the Word," ...and had in mind "a musical exegesis that might intensify the Biblical text through melodic, rhythmic, harmonic, and contrapuntal means and might thus let it strike the hearer in full force." 
|||Johann Sebastian Bach and Liturgical Life In Leipzig, Stiller, Guenther (trans. Bouman, H. A., Daniel Poellot, Hilton Oswald), Concordia, 1970.|