For authors of fiction, how much may be considered too much in writing about Christianity and its history? Would there be a point at which what an author depicts in a book is sacrilege?
Sure, but that doesn’t mean an author has thereby committed sacrilege because there is a difference between depicting sacrilege and committing sacrilege. An author may depict a situation in which characters in his fiction act sacrilegiously, but that does not mean he has personally committed sacrilege. It would depend on whether or not he was intending to endorse what his characters were doing.
For example, if anti-Catholic characters in a novel on the French Revolution are depicted as disinterring saints’ graves and tossing the saints’ remains into a common grave, the author is not committing sacrilege by depicting characters who commit sacrilege against the saints’ remains. But if the author is presenting as historical fact something that is not historical fact, and does so in order to call into question Christian belief in what has been commonly accepted as historical fact and orthodox Christian dogma (e.g., Jesus depicted as married to Mary Magdalene in Dan Brown’s The Da Vinci Code), then, yes, a case could be made that the author has committed sacrilege.