In popular conversation a “diatribe” is when someone really goes off on another person, with ranting, raving and condemnations. It is not done in polite company. So when you hear that Isaiah 40, or any other passage, is a diatribe, take a breath. In literature “diatribe” refers to a type of discourse. It involves an intense “dialogue” between the speaker and a real or imagined opponent.
the term “diatribe” means something like “wearing away”. So with questions and argument, a series of arguments are brought to bear on the topic in rapid succession.
Isaiah 40:12-31 has by my count 15 questions (this will depend on how the translator will divide sentences). They break out into 5 main questions. Which look something like this:
1. Who made Heaven & Earth, Justice & Knowledge? v. 12-17
2. What is God like? v. 18-20
3. Haven’t you been paying attention? v. 21-24
4. Again, what is God like? v. 25-26
5. Why do you despair? v. 27-31
Alec Motyer in his commentary (The Prophecy of Isaiah, IVP) says that v. 27’s complaint is the centerpiece. In a time of national calamity the faithful are asking if they are hidden from or forgotten by God. The “diatribe” form is a sort of shock therapy – reviewing what they ought to know and applying it to their situation.