The author builds his case that the reason why so many new converts (he actually says 80-90% of converts from crusades) fall away from their new faith is that they were not told the whole counsel of God as to why they should become Christians. They were told that they should try Christianity because it will make their lives better; it will help them achieve peace and have a happier marriage and all the rest. But they weren't told what the Bible teaches: that the only way to escape the wrath to come is to embrace Christ who will take their punishment for them. So when things don't go the way they had hoped, they are disillusioned and embittered toward Christianity for not delivering what was promised. Telling people that they stand condemned before the Law of God is not pleasant, but that is the only way people will see their need for a Saviour.
This book is better that I thought it would be. It is thought provoking and convicting (for me anyways) as it goes beyond teaching the function of the Biblical Law, to speaking to Christians who are not demonstrating a heart for the lost by engaging in personal evangelism. We have a thousand excuses, don't we, for not getting into the game as it were. The reasons can be pretty strong and deep rooted, and should drive people such as myself to my knees in prayer for deliverance from my fear and inactivity.