We can't really say the Christian tradition must be divinely inspired because it produces social stability and personal happiness and turns people's lives around, getting them off drugs or alcohol or whatever, The reason for this is that other religions like Islam, Mormonism and Scientology also seem to do all of these things. And we also can't say the Bible must be divinely inspired because it moves people, is so beautifully written, changes lives etc, since many other books do these things too. That's why I chose the criteria of amazing predictions and astounding knowledge for divine inspiration, and indeed this is why many fundamentalist Muslims and Christians use these criteria. They know they need to try to find something that's unique to their tradition.
How do we know all those religions aren't divinely inspired? Each religion claims to be the only true religion, but maybe they are wrong about being unique and divinely inspired in other areas that matter more?
Most Christians are aware that the entire package of Christianity contains many mistakes, but they overlook these mistakes because they don't want to risk losing the truths of Christianity in the process of extracting the mistakes. It's like the parable of the wheat and the tares:
http://www.biblegateway.com/passage/?search=Matthew+13:24-30&version=NKJV
So if Christianity helps Plantinga in real ways then isn't he being rational to accept some harmless non-falsifiable lies as part of the package? (I don't know if this is rational by the philosophical definition, but isn't it understandable?)