The story goes that Dwynwen lived in the 5th Century AD. She was one of 24 daughters of St Brychan. She fell in love with a young man called Maelon. Stories differ substantially on the events that followed. Either she is raped by Maelon and prays for assistance, or she is unable to marry him due to her father's refusal and prays to forget her love for him. God apparently sent an angel bearing a potion for Maelon to drink. After drinking it, Maelon turns to ice. As well as receiving the potion, Dwynwen is allowed 3 requests from God. These three requests are that Maelon be released; that, through her, God look after all true lovers; and that she remain unmarried. At this point, she retreats to the solitude of Llanddwyn Island off the west coast of Anglesey to become a hermit until she dies c 460 AD. The site is still visited today.
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.