Being in St David's Day mode my mind has gone back to St David's Day as a child in Junior School, when it was always a half day at school (we'd spend the morning drawing something Welsh - a leek or a daffodil - the girls would wear traditional clothes with black top hats you could buy from local shops). We'd also get a half day if there was a rugby game on (my dad was a soccer man and would sometimes moan that they didn't do that for soccer). Anyway on the afternoon of December 6, 1967 (I was 8) my dad took me to Rodney Parade in Newport to watch Monmouthshire play the touring New Zealanders (who only lost one game that tour). The great Keith Jarrett (not the jazz pianist) whose career was tragically cut short by strokes when only 25, was playing, and Monmouthshire did well thanks to his kicking, eventually losing 23-12. All I remember is Jarrett kicking a lot of goals and that they wore black shorts. It might have been the only rugby game my dad took me to (not sure how he had time off work).
Bonhoeffer uses a similar phrase 'worldly Christianity'. It's J Gresham Machen that I want to line up most closely with. See his Christianity and culture here. Having done commentaries on Proverbs (Heavenly Wisdom) and Song of Songs (Heavenly Love), a matching title for Ecclesiastes would be Heavenly Worldliness. For my stance on worldliness, see 3 posts here.