You'll need to double click to see this but it is an advert for t-shirts on the official Jan Akkerman website. It says t-shirts are available in various sizes but the xxxl have all sold out. Given that most Akkerman fans are over 50 in age if not waist size that comes as no surprise and made me chuckle.
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.