What Does Goodbye Mean? Goodbye is a shortened and corrupted form of "God be with you". The expression has had in its long life many spellings, of which "God be why you" and "Goodbye" are representative. God became good probably by association with Good morning, Good evening, and Good day which were probably themselves shortened forms of "God give you good morning, etc". The suggestion has been made that Goodbye is a corruption of "God buy you" but the earliest forms of the salutation show that this is not the case. Many common exclamations, such as bless you, save us, and preserve us used formerly to begin with God. Mercy on us is an abbreviation of "God the Lord have mercy on us".
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.