For those who read music having the printed score available as the performance unfolds helps us see the wonder of the way this magnificent piece is constructed.
The Prelude is comprised of three distinctly different themes introduced in succession, then reintroduced and developed with differing harmonies and textures. It ends with the same thematic material with which it began but harmonically altered to add further poignancy and drama to the conclusion.
The Fugue is made up of three interconnected sections each a separate, fully developed fugue unto itself. If you don't know what a "fugue" is, please just listen, and it should reveal itself to you.
The sturdy, relatively-simple first section is built on the opening phrase of the hymn tune called "St. Anne," which most of us know today as "O, God, Our Help In Ages Past."
The second section of the fugue introduces a lively running figure developed against itself in counterpoint combined with the St. Anne hymn tune introduced in the first section.
The third section introduces yet-another strikingly different, highly energetic theme, then develops it in counterpoint with material from the second section. The work is brought to a glorious conclusion of unsurpassed grandeur and magnificence when the St. Anne theme from the first section is reintroduced in the bass as a powerful underpinning thus combining all three thematic elements so brilliantly it could only be described as breathtaking.
It has been suggested that the tripartite (three-part) structure of both the prelude and the fugue represents the Holy Trinity. Whether Bach consciously intended this symbolism or not, surely there could be no clearer demonstration of three different elements working interdependently to form a whole greater than any of its parts than the St. Anne Fugue.
I've listened to this work hundreds of times over several decades. It has always been a humbling, awe-inspiring experience that never fails to raise goose bumps, and it remains so to this very day.