http://en.wikipedia.org/wiki/Mortal_sin#Roman_Catholicism

If you look at the three conditions that a sin must meet to be considered a mortal sin, I think there is a very high probability that writing a book titled God Is Not Great is a mortal sin. There are hundreds of things that Christopher did which very likely meet those conditions, and there is also a high probability that he has not repented about them before death.

According to Catholicism people do not get into heaven by living according to what they sincerely believe, and many, many, Catholic scholars would say that Christopher is bound for hell.

http://en.wikipedia.org/wiki/Mortal_sin

1. Its subject must be a grave (or serious) matter.

2. It must be committed with full knowledge, both of the sin and of the gravity of the offense (no one is considered ignorant of the principles of the moral law, which are inborn as part of human knowledge, but these principles can be misunderstood in a particular context).

3. It must be committed with deliberate and complete consent, enough for it to have been a personal decision to commit the sin.

The question is about point 2. While Hitchens knew that it would be morally unacceptable according to Catholic doctrine, he considered writing the book as the right thing to do according to his own moral standard, so he would not be doing something he considered as a gravely wrong thing to do. Similarly for other people who live according to what they sincerely believe: they are not committing actions with full knowledge that the actions are the wrong thing to do.

It does leave room for interpretation, and can be argued both ways, but I do not wish to be drawn further into a religious discussion.