“Durch Adams Fall ist ganz verderbt,” BWV 637, is one of the most famous preludes in the Orgelbüchlein. Partially, that’s because the piece is dramatic and shocking, and it’s easy to appreciate what’s going on from the title alone. This is also one of the few Orgelbüchlein chorales for which we can point to a direct compositional model for Bach.

The idea of the “Fall” is easy to illustrate. Dietrich Buxtehude did it in his chorale prelude on the same tune – look at the pedal line:

and later on, Buxtehude used some conventional chromaticism, to express the idea of a lament:

Bach famously walked 250 miles, from Arnstadt to Lübeck, to learn from Buxtehude and “comprehend one thing and another about his art.” While there, he likely copied and acquired all of the organ music he could get his hands on, to the extent that Bach and his later students, far away in Central Germany, were responsible for preserving many of Buxtehude’s compositions that would otherwise have been lost. Here’s the opening of Bach’s prelude. He uses falling pedal motion and chromatic writing at the same time, combining two of Buxtehude’s ideas:

In Buxtehude’s prelude, the pedal line falls in diatonic fifths; you can point it out and savor how literally he interprets the text. But in Bach’s setting, the pedal intervals are chromatic, and they’re wild – as you go through the piece, you find include sevenths of all descriptions: major, minor, and diminished. This isn’t a mere tumble; this is the sort of fall that causes you to break an ankle.
Bach takes his cue from the first two lines of the chorale text: “Durch Adams Fall ist ganz verderbt / Menschlich Natur und Wesen” (Through Adam’s fall human nature and character is completely corrupted). It’s even been suggested that the chromatic inner voices represent the wriggling of the snake. The sound of the piece is highly disturbing; Bach doesn’t spare his listener. If the text suggests the image of something “completely corrupted,” Bach has no problem writing music that sounds completely corrupt and rotten.
Here’s a performance by Marcel Dupré (1886–1971), one of the most formidable organists of the 20th century. Dupré’s way of playing Bach, with seamless legato and deliberate tempos, sounds dated now, and some of his playing can sound ice-cold and emotionless. But he had the technique and control of a Horowitz, and even his oddities can make fascinating listening. He’s playing the large Cavaillé-Coll organ of St.-Sulpice in Paris: