The work in question is the Prelude in C Major from The Well-Tempered Clavier. This well-known composition was adapted by the French composer Charles Gounod (1818-1893) as the accompaniment to his celebrated Ave Maria. However, the Prelude as Bach wrote is a masterpiece in its own right. I begin to learn to play this piece quite a number of years ago, during my teenage years. As a beginning pianist, the work was (and even today, is, to some extent) a challenge to play, mainly due to the need to control one's fingers so that the rhythm of the piece remains constant. Nevertheless, the effort of learning to play it was also quite satisfying. In form, the prelude is disarmingly simple. It consists of a series of arpeggiated chords which begin in the key of C major, progress through a number of different keys, and then return to C major. However, within this rather simple form, Bach is able to create a great sense of drama, ratcheting up the tension through the various key changes as the work approaches its climax and then comes to a calm closing in the original key. There is no "melody" per se, but a very strong sense of horizontal movement in the work, a sort of implied melody, perhaps. I might add that, having once had the chance to play the work on a harpsichord (probably the instrument Bach originally wrote it for), it seems equally effective on that instrument and the modern piano--another proof of Bach's consummate skill as a composer.
To me, the way in which Bach takes such relatively simple elements and creates a work of great power within a relatively short time frame (the work is only a few minutes in length) is a striking example of his great genius. How much more so is his genius evidenced in his longer and more complex compositions! It is not surprising to me, therefore, that in an article (see here) presenting twenty arguments for the existence of God, the Catholic philosopher Peter Kreeft wrote the following:
There is the music of Johann Sebastian Bach.
Therefore there must be a God.
You either see this one or you don't.
I don't know about others, but I certainly "see this one"!
Image of Johann Sebastian Bach from commons.wikimedia.org