Perhaps this reference might have it: http://www.amazon.com/New-Bach-Reader-Sebastian-Documents/dp/0393045587 It says, quote, "A wonderful collection of letters by and articles about Bach, both from his own ... These are among the best known of Bach's letters, and are a fairly good ..." Source: http://www.google.com/search?q=Letters+written+by+Bach%22&sourceid=ie7&rls=com.microsoft:en-us:IE-SearchBox&ie=&oe= +5

1) "Like a string of pearls adorning the neck of a lady, nine canons grace Bach's Aria with Thirty Variations, giving them mystery as well as form and substance. "Prepared for the enjoyment of music lovers" they have indeed made joyful generations of thankful musicians. Yet through these pleasurable tones the thoughtful ear faintly hears echoes of an earlier note humbly scored in his Bible and with his hand: "splendid proof," wrote Bach, "that besides other arrangements of the service of worship, music too was especially ordered by God's spirit through David." All music--choral, orchestral, sacred, secular, vocal or Clavier--for Bach there appears to have been but one Übung--one order--that which was decreed by the Spirit of God. In the esoteric symbols of his set Johann Sebastian participates not only in the spirit of his age, but also the spirit of creative ingenuity, and in the Spirit of his God." "Bach wrote these words in the margin of his recently discovered Bible Commentary by Lutheran theologian Abraham Calov. The marginal notation refers to portions of the twenty-fifth chapter of I Chronicles." Source and further information: http://jan.ucc.nau.edu/~tas3/intentional.html 2) "Bach bought a three-volume Bible commentary by the Lutheran divine Abraham Calov in 1733. It has survived, with his annotations, to our day. These notes show Bach pondering the biblical warrant for church music, which Calov traced to King David’s appointment of three “guilds of Levitcal musicians” (John Kleinig, “Bach, Chronicles, and Church Music,” in Logia: A Journal of Lutheran Theology 9:3 [Holy Trinity, 2000], pp. 7–10). Calov wrote that these musicians “were to turn God’s word into spiritual songs and psalms and sing them at the temple set to the accompaniment of music played on instruments.” Bach marked the passage, and wrote, “N. B. This chapter is the true foundation for all God-pleasing church music.” Elsewhere in his copy of Calov, Bach wrote that “together with the other arrangements for the divine service [liturgy], music too was instituted by God’s Spirit through David.” Bach was confident in the rightness and even necessity of such proclamation. Lutherans place great emphasis on God working specifically through the “means of grace,” i.e., Word and Sacrament; Bach understood the sung word, and not only the preached word given in the sermon, to be included here, when he wrote, “In a reverent performance of music God is always present with His grace.”" Source and further information: http://www.touchstonemag.com/archives/article.php?id=15-02-042-b 3) "Bach's copy of a two volume Bible commentary by the orthodox Lutheran theologian, Abraham Calov, was discovered in the 1950s in a barn in Minnesota in the US, purchased apparently in Germany as part of a "job lot" of old books and brought to America by an immigrant. Its provenance was verified and it was subsequently deposited in the rare book holdings of Concordia Seminary in St. Louis, Missouri. It contains his markings of texts for his cantatas and notes. It is only rarely displayed to the public. A study of the so-called Bach Bible was prepared by Robin Leaver, titled J.S. Bach and Scripture: Glosses from the Calov Bible Commentary (St. Louis: Concordia Publishing House, 1985)." Source and further information: http://en.wikipedia.org/wiki/Johann_Sebastian_Bach 4) Text in German original: "Ein herrlicher Beweiß, daß neben anderen Anstalten des Gottesdienstes, besonders auch die Musica von Gottes Geist durch David mit angeordnet worden." Source and further information: http://www.knaw.nl/publicaties/pdf/20041092.pdf

Try to look at wikipedia.org