The book has some virtue in stressing how important it is to represent God and the Kingdom of God. But it has a grave theological flaw common to much of American Christianity.
Scripturally, Yeshua saves us from slavery to sin. Before we have faith in Yeshua we are "stuck" being unable to live as virtuously as we would like. Our willpower will never be enough, no matter how much we make deals with ourself or ask God to strengthen it. There is no outside help from God available because God will not send his Spirit into someone enslaved to sin. In Isaiah's words,
Behold, Adonai's hand is not shortened, that it can't save. Neither his ear heavy, that it can't hear. But your iniquities are a separation between you and your God. And your sins have hidden his face from you, so that he will not hear.Faith in Yeshua allows us to escape slavery to sin. Then God can send his Spirit into us. The apostles call this "making us innocent" (dikaiosune). Just as the word "innocent" in English means both not being guilty and being pure inside, in Greek the same two meanings apply. We are legally in a better position before God, and also able to be pure.
(Note: the word dikaiosune is usually translated "justified", which is accurate but often not meaningful to people, or "righteousness", which is a very bad translation.)
Despite what is often taught, Yesuha's sacrifice does not make our crimes forgiven. Rather, our repentance and faith in Yeshua allow him to forgive us. (If you doubt this, read First John 1:7-10 again.)
Here's a metaphor that came to me this weekend, while discussing theology after Shabbat services. Like all metaphors it has its weaknesses, but it might help here:
Imagine you are in a different country with slightly different traffic laws, and a king.In the metaphor, our faith in Yeshua's sacrifice is like gaining the driver's license. Our repentance is like subsequently dealing with the tickets.
You don't have a driver's license. No matter how well you drive, you cannot do so legally. The king cannot hire you to work for the government as a courier, or in other ways treat you as a driver without compromising his own justness.
Also, you don't drive perfectly. You have earned a few driving tickets.
Later, you get your driver's license. Now you can drive legally! The king, and everyone else, can relate to you as a legal driver. But you still have those tickets.
You do what is necessary to deal with the tickets, and finally have a clean slate as a legal driver.
My main point is that we are saved from slavery to sin. In particular, we are not saved from God's wrath. God is not angry and eager to punish sinners. Scripture describes God as prefering mercy and hesitant to punish. But he is a just God, and will not compromise his justness. He will only extend so much mercy to people still enslaved to sin.
(Think of how you would be more willing to be forgiving, and more just in acting forgiving, to a friend or family member who got his or her alcoholism under control, than if that person kept getting drunk and into trouble.)
So Ray Comfort's book might inspire followers of Yeshua to do more for God's Kingdom. But it does some harm in perpetuating false theology about being "saved from God's wrath".
In particular, it would be a terrible manual for how to talk about Yeshua to unbelieving Jews. Jewish people know quite well that God is not uptight and wrathful. When a Christian tells an unbelieving Jewish person that he or she needs to escape God's wrath, that only reinforces in the Jewish person's mind the idea that Christians and Jews must be worshipping two very different Gods, even though the Christian says otherwise.