I notice in my sidebar that my most popular post at present is one I did in 2008 on musicians who died aged 27. See here. No prizes for guessing why people have been looking at it. Tragically, another 27 year old musician died last weekend. There is probably nothing over significant in the number. It allows time for real success but is still tragically young. Others, like Marc Bolan, get beyond 27, but still die tragically young.
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.