Eleri put these five pieces of embroidery up recently. They were kindly done by her mother over the years. They reveal that Rhodri is the oldest, Dylan was born earliest in the calendar year, Dewi earliest in the academic year and Gwion was the heaviest. I would expect that Owain will be the tallest.
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.