Cracking the Bach Code

J.S. Bach possessed an amazingly comprehensive musical "toolbox."  If, say, he was trying to get across the idea of the Holy Spirit's Pentecost descending, the crucifixion's agony, or the love of Jesus for the Christian, he took out all 64 musical Crayolas and went to work associating certain elements of the piece with Biblical concepts.  The incredible thing:  subconsciously, it did and still does carry across the meaning of the work effectively.

Some things I keep in mind while listening and studying Bach (that you can use, too!):

1.  What is the instrumentation?  Oboes usually denote a countrylike, shepherd atmosphere; trumpets and timpani accompany festival pieces and also mean it is in D Major (the sunny, happy key in which they could play).  Sometimes, the basso continuo or plucked strings can act as a heart pulse or walking rhythm.  Groups of three might signify the Trinity.
2.  What vocal parts are employed?  Sopranos, originally boy trebles, are the Christian nature of the soul.  Altos generally sing about repentance and God's forgiveness.  Evangelists and other heroic characters are played by tenors, and basses often represent Jesus.  Soprano and bass duets typically act as conversations between the Christian and the Lord.
3.  What key is it?  That sounds really picky, but to the Baroque people and their doctrine of affections associated with each tonality, this was extremely meaningful.  A key like A or E-flat Major might hint at the Trinity.
4.  What individual genre does the piece employ?  Almost all of Bach's pieces fit under some component of the dance suite structurally.  Gigues usually indicate confidence and lightheartedness about the topic - a 12/8 pastorale like the opening of BWV 244 hints at the Lamb of God.  Others could be sarabandes (very slow), minuets (the precursor of the waltz, in 3/4), gavottes (in 4/4 with an upbeat on 3), or airs (the catch-all category :) )
5.  What is the shape of the melody?  Does it draw "crosses" with chromaticism on the score?  Does it descend to earth or ascend to heaven?  Bouncy, joyful leaps or gradual stepwise motion?  A twisting, "wandering" feeling? For instance, the Cross Motif:

6.  Of course, where does it fit in the church year?  What is the main idea of the hymn of the day?  If the chorale choice seems somewhat unusual, how could Bach have connected that with the Scriptural readings?
7. Numerically, are there any patterns?  In Baroque times, letters where associated with numbers; "Bach" adds up to 14 and "J.S. Bach" equals 41 in this system.  During any given piece, he seems to have a preoccupation with what goes on in those respective measures, as well as including them in other ways.  On top of that, he likes putting in significant moments at important mathematical divisions like the halfway-point or third.

Once you've scratched the surface, it's kind of hard to stop.  There are lots of great books and papers out there written on the subject, especially the St. Matthew Passion.  The Emmanuel Bach Choir (emmanuelmusic.com) has great, concise notes for every cantata and motet, and bach-cantatas.com contains a variety of resources and discussions.  Have fun!