I read an interview with him about this book (I think also NPR). He was quite clear that in spite of his lack of belief in it, he thinks Christians have done a lot of good for the world, he even went so far to separate the Christians from the faux-Christians (groups like Westboro Baptist Church).
Also keep in mind that when he is on his TV show, he's a performer. Humble performers are boring. Just imagine his show if it was called "I humbly disagree", and after examining the issues he'd just say "Yeah I don't think it's true, but whatever". I guarantee you a 0 in the ratings.