What is the idiom in alas, babylon and where is it found

"Alas, Babylon" is the quote that serves as the title of the novel. Years before, Randy and Mark agreed that should Mark get wind of an approaching nuclear attack, he would warn Randy in some way by passing along the words "Alas, Babylon," because it would be risky to say the truth over telephone or telegram. This quote has biblical origins; in the Bible, Babylon was a city of sin destroyed by God. It is fitting because many cities in the U.S. are similarly destroyed but by nuclear attacks. Babylon" is the quote that fills in as the title of the novel. A long time sometime recently, Randy and Mark concurred that ought to Mark get a twist of a moving toward atomic assault, he would caution Randy somehow by going along the words "Too bad, Babylon," since it is dangerous to state reality over phone or message. This quote has scriptural sources; in the Bible, Babylon was a city of transgression wrecked by God. It is fitting, on the grounds that numerous urban areas in the U.S. are likewise decimated, yet by atomic assaults.

The answer would be logical.