What is the author's purpose in the nonfiction book, The Prophets?

The writer, a Jewish theologist, wrote the book with the desire to persuade and inform the reader about the Old Testament prophets and their relationship to God. When Heschel begins, he tries to stay informative more than persuasive, but as the writing continues the reader finds that he begins to give his opinions. He also gives the reader reasons why he is correct and supports his own opinions by giving the reader quotes from other theologists and writers.