The premise of this book is that, embedded in most, if not all cultures of the world, are "cultural keys" which prepare them to receive the gospel.
These include belief in a 'sky God', scapegoat-concept of sin-bearing, new birth rituals, and an expectation of people who will bring a book.
He also personally found cultural keys in a group in New Guinea where he and his wife were missionaries. Their concept of a Peace Child opened their hearts to hear the gospel.
Although there is merit in such a concept, as a way to contextualize the gospel, it is not absolutely necessary as a precursor to preaching the gospel. The gospel is powerful enough on its' own, and is able to soften the hearts of lost men and women.