Bell didn't say it, but Bach's "Chaconne" is also considered one of the most difficult violin pieces to master. Many try; few succeed. It's exhaustingly long -- 14 minutes -- and consists entirely of a single, succinct musical progression repeated in dozens of variations to create a dauntingly complex architecture of sound. Composed around 1720, on the eve of the European Enlightenment, it is said to be a celebration of the breadth of human possibility.
If Bell's encomium to "Chaconne" seems overly effusive, consider this from the 19th-century composer Johannes Brahms, in a letter to Clara Schumann: "On one stave, for a small instrument, the man writes a whole world of the deepest thoughts and most powerful feelings. If I imagined that I could have created, even conceived the piece, I am quite certain that the excess of excitement and earth-shattering experience would have driven me out of my mind."
So, that's the piece Bell started with.