The first words I hear, arriving at this year’s Greenbelt festival, seem to sum it all up pretty well. They’re spoken by a girl aged about 12 to her friend: “Christian stuff,” she is saying, “can be fun.” And that certainly seems to have been the ambition, in the sun-drenched field that was Greenbelt 2017: music, dance performances, theatre, art workshops, movies, authors’ talks, yoga, storytelling, tree-climbing, football.