Stephen Nichols writes in For Us and For Our Salvation: The Doctrine of Christ in the Early Church:
When Johann Sebastian Bach composed the music for the section on the Creed in his Mass in B Minor, he revealed his skill not just at music, but also at theology. He used full voice choirs in his musical telling of the Nicene Creed, except in two pieces. In one, “Et in Spiritum Sanctum” [“and in the Holy Spirit”], treating the final clauses of the Creed, which relate to the Holy Spirit, he employed an aria with a solo bass voice. In another, “Et in unum Dominum” [“and in one Lord”], treating the Creed’s central points about Christ, he has a duet, with soprano and alto voices gracefully extolling belief in the deity and the humanity of Christ. Bach has each voice sing the Christ-centered phrases of the Creed in succession before weaving them together in one unified chorus. Bach was musically doing for the Nicene Creed what the Chalcedonian Creed would do for it theologically: bring the two natures of Christ, his full humanity and his full deity, into one undivided, unconfused, and unmixed person. Chalcedon and Bach tell us there are two distinct and individual natures forming one unified person, two voices forming one grand choir. (99)
Nichols suggests listening to the Berlin Philharmonic’s 1974 recording, conducted by Herbert Von Karajan and available on the record label Deutsche Grammaphon.
For another nice resource on Bach’s faith, see this video from Speaking of Faith‘s website. It’s about three and a half minutes and talks about how Bach’s Bible notes prove that he believed what he wrote in his music.