From a NY Review of Books piece by George B. Stauffer (“Why Bach Moves Us“)
.. Bach’s identification with Martin Luther that made all the difference. Luther’s earthy German translation of the Bible, a “prose of the people,” provided Bach with bold images to paint in music. It was the perfect counterpart to Luther’s hymns and hymn texts, to which Bach returned time and time again. Luther’s advocacy of music and his conviction that it could make scripture come alive legitimized Bach’s compositional ambitions.