Upon finishing the book of Isaiah a couple of days ago, I'm reminded of my favorite seminary professor, Dr. John Saihamer. He's the one that helped me see that the Old Testament's message is the same as the New Testament's. In fact he used to call it "The Same Ole Testament." Here's what he says concerning reading the prophets:
"The prophets did not write their books to teach their readers the Sinai covenant. Their intent, like Moses’s, was to call their readers to a life of faith under the new covenant" (Isa. 7:9) (John Sailhamer, "Preaching Through the Old Testament" pg. 120).