 For incidental music to a New York production of "Ben-Hur" in 1899, he based his composition on Greek modes.This music was to go on to become his most popular work; it is said to have been performed some five thousand times in English-speaking countries by 1930.Kelley's best-known composition was an oratorio, "The Pilgrim's Progress", composed on a text by Elizabeth Hodgkinson and based on the eponymous text by John Bunyan.