O Mensch, bewein dein Sünde gross (organ chorale prelude, BWV 622)

In this hymn prelude about bewailing our sins, Bach chooses not to set the familiar melody starkly and austerely. Instead, the most flowery and passionate ornaments decorate the song. Organists know that our moment is going to be at the end: the famous c-flat major chord which strikes the word "Kreuze" (cross) and the following twists to the slow end on "lange" (long). Let's explore how these harmonies are not randomly chosen but are instead striking alterations of a regular harmonic progression. It's not hard to see why this chorale prelude has long been one of Bach's most popular.
 
Thanks to aMoB listener Dan for the suggestion!
Erwin Wiersinga plays the organ chorale prelude for the Netherlands Bach Society All of Bach project: https://www.youtube.com/watch?v=5oha0oITfYk