Created relatively late in Hockney's career, A Bigger Message is a culmination of a series of works by Hockney inspired by The Sermon on the Mount, Claude Lorrain's 1656 painting. Lorrain was one of Hockney's heroes - a French Baroque landscape painter (in English, simply "Claude"), known for revolutionizing the genre and basing his work on observation, Claude has had a strong effect on Hockney's landscapes. In order to create the painting, Hockney spent 3 weeks digitally cleaning the painting by Claude on his computer. Through this process, Hockney got to know the composition better, and created a thoroughly contemporary way of painting; rather than working from life, or even from the original work, his inspiration came from a mediated, doctored version of the Old Master's work.
Hockney is fully aware that many art enthusiasts would frown on this process, and fully intends to tweak the nose of tradition. He is also, however, following in the footsteps of another renegade, Picasso, who painted Cubist versions of Velasquez's Las Meninas based in part on reproductions from newspapers and magazines. Distance from the original allows the artist to create his own spin on the scene. Hockney takes his palette not from Claude but from Pop Art, and from his own earlier depictions of the Californian and Yorkshire landscapes. He draws more attention to the human figures in the foreground of the image, and represents the mountain as an oversized red rock, imbuing the scene with a distinct sense of drama absent from the original.
(http://www.theartstory.org/artist-hockney-david-artworks.htm#pnt_7)