For a book that's pretty solidly rooted in academic lecturing, Dan Brown goes all mystical at the end:
Like the murmurs of spirits in the darkness, forgotten words echoed. The quest for the Holy Grail is the quest to kneel before the bones of Mary Magdalene. A journey to pray at the feet of the outcast one.
With a sudden upwelling of reverence, Robert Langdon fell to his knees.
For a moment, he thought he heard a woman's voice…the wisdom of the ages…whispering up from the chasms of the earth. (Epilogue.48-50)
Whoa. Dang. That's quite the tone shift from guns-and-murderous-monks-and-car-chases.
This ending achieves a couple of things for the reader. First of all, it provides some closure to the story. Admit it: you were a little disappointed when you thought the book was going to end without Langdon finding the Holy Grail. (Couldn't Sophie's grandmother have thrown him a bone?)
But it also opens up the ending to make you wonder. Did he really hear a woman's voice? Was it Mary Magdalene? Was it a vagrant down the street? Is he having a psychotic break? What was the wisdom of the ages?
Guess you'll have to read the sequels to find out.