The taxi takes Sophie and Langdon on the fastest route to their address, which is through the Bois de Boulogne—a park known to be home to "freaks and fetishists" for hire (a.k.a.: lots and lots of prostitutes of all shapes, sizes, and specialties).
Langdon distracts himself by giving Sophie a brief history of the Priory of Sion:
It was founded in Jerusalem in 1099 by a French king, Godefroi, who possessed a powerful secret that had been in his family since the time of Christ.
He worried his secret would be lost upon his death, so he started the brotherhood to protect and pass down his secret through the generations.
Then the Priory learned of a stash of hidden documents that corroborated Godefroi's secret, so they vowed to recover them and protect them forever.
In order to retrieve the documents, they created the Knights Templar. (Apparently, protecting pilgrims—their more popularly known mission—was just the guise that allowed them to operate under the radar.)
No one knows if they ever got the documents, but whatever they found made them super rich and powerful.
Basically, the Knights located themselves very strategically during the Second Crusade in the Holy Land, and were able to house themselves in a stable under the ruins of Solomon's Temple—which apparently was above a sacred chamber known as the Holy of Holies.
For nine years they lived there and excavated in secret, but then they found whatever they found and took it to Europe, where all of a sudden they were really influential.
Because of this secret something, Pope Innocent II issued them an unprecedented papal bull that gave them limitless power and declared them "a law unto themselves."
This gave them the power to expand rapidly and set themselves up as important creditors to kingdoms and royals.
Then, in the 1300's, Pope Clement V decided that the Knights Templar were way too powerful. So he issued secret sealed orders to kill all the Knights all across Europe on Friday, October 13, 1307. This worked—really well—and they were pretty much obliterated.
Since then the few Knights that survived went underground and took cover in the Priory of Sion, who smuggled the powerful documents onto Templar ships…where they were then lost to history. There's speculation the documents have been moved several times, and may now reside in the United Kingdom, but no one really knows for sure.
Since then, the collection of documents has been dubbed the "Sangreal," which is also known as the Holy Grail.