Actually, Kushner spends most of the book not answering the question the title of his book poses, but rather fills the pages telling us how good we don't have to be. According to Kushner, we don't have to be perfect, and contrary to the voice of our conscience God doesn't require perfection from us. Kushner gives no Scriptural justification for this. One is struck with the impression that throughout the book Kushner is merely giving us his own ideas while trying to give the appearance that he is getting such ideas out of the Bible. But in reality he does violence to the Scripture and attempts no real exegesis.
Kushner finally gives an extremely vague answer about how good we have to be - so vague it is almost impossible to relate. Was it that our good deeds must outweigh our bad? Or was it that we must only try to be good? It's hard to say. His answer is unclear, but this is basically the gist of what he points us to: God doesn't require perfection, He just requires you to do the best you can. But how do you know when you've done your best? Who knows.
Such an answer does three things: 1) it violates the teaching of Scripture, 2) it leaves people in inevitable uncertainty about the state of their soul, and 3) it keeps the door open for hypocritical and vain self-righteousness.
I strongly reject the conclusions of this book and encourage those who read this to find out from the Bible itself what God has to say about how good we have to be. Don't be deceived by false teachers who tell you that God requires less than perfection. According to the law of God moral perfection is indeed required (Deut. 4:1-2, 6:4, 25, 18:13, 29:29). Even though none one of us is morally perfect, this is only because each one of us is inexcusably evil. Yet the greatest news in the Bible is that Almighty God loves us sinners and atoned for our sins through the death of the Messiah and His Son, Yeshua. By putting our trust in His atonement we are forgiven and our sins will not condemn us. Only this honors the perfect law of God and shows us the amazing love and mercy of God. God does not compromise His justice, but satisfies it by the cross, which enables Him to justly forgive and justify sinners. This is the truth about God's judgment, and I urge everyone to believe it before it is too late.
Contrary to what Kushner says, we have to be morally perfect in order to be acceptable to God. This is precisely why we need salvation through the holy sacrifice of Yeshua the Messiah.