Dr. Thomas R. Rossin wrote his doctoral dissertation on the over 400 of notes Bach made in his "Calovi Schriften" Bible.
Dr. Rossin wrote:
"Only three percent of the markings relate to music, and there's only one reference to a specific piece of music.
Otherwise we learn about the man Bach, his personality, and that he really was believing what he was writing, that his music was not just a reflection of his job ...
He was employed by the church to write church music, so of course he was going to write that (he was an orthodox Lutheran.)
But now we have proof - things that Bach wrote in there point directly to his personal faith. He wrote for his own edification, not for anyone else to read ...
No longer can we have purely academic performances where all the notes are lined up right and the phrases are beautiful and it's done musically.
It has to be more than that. One must understand the theology in order to perform the music well."