What is the major plot twist in Chapters 96 through 100?

The major plot twist in Chapters 96 through 100, is that the Teacher is Leigh Teabing. The author does a superb job leading the reader to believe that Bezu Fache was the Teacher. However, when Teabing ambushes Sophie and Robert, he reveals his identity and role in the plot. They learn that it was Teabing that murdered Sophie's grandfather because he believed Jacques sold out to the Church, had succumbed to their influence, and decided not to reveal the truth of the Grail to the world.