Today, musicians regard Bach's six cello suites as the most popular in the world. They didn't always have this prestigious reputation though. Pablo Casals spent years practicing the pieces, which at the time were viewed as technical warmup exercises. Eventually, he became the first musician to perform them in public. When asked about the importance of the pieces, the cellist Lluís Claret said, "It's like our bible."  After Casals recorded his interpretation of the pieces in 1936, the pieces were regarded as works of art necessary for any cellist to be seriously considered a master of the instrument. Casals changed the entire culture of the cello and is thus the greatest cellist of all time.
While Casals did have a significant impact on the cello community, this impact alone is not enough for him to be considered the greatest cellist of all time. He merely laid the foundations for the greatest cellists of all time by popularizing the Bach suites.
[P1] Casals popularized Bach's six cello suites, opening the music community's eyes to their excellence. [P2] Ultimately, this changed the world of professional cello players. [P3] For this reason, Casals is the greatest cellist of all time.