How true should you be to your characters?

This is a question I've been asking myself for several months now in light of a new project I'm working on. You can read the first four chapters by clicking on the link in the sidebar. A Familiar Evil is undoubtedly one of the toughest stories I've attempted to write. My main characters are not Christians and are currently separated in marriage. I don't want to offend my readers, but at the same time I feel (after careful prayer and consideration) that I must be true to my characters. So ther...