How we cite our quotes: (Chapter.Paragraph)
Quote #4
"They are dead," the monk said. "All four of them. And they have played me for a fool. Tell me where the keystone is."
"I don't know!" Sister Sandrine said truthfully. "That secret is guarded by others." Others who are dead!
[…]
As Sister Sandrine fell, her last feeling was an overwhelming sense of foreboding.
All four are dead.
The precious truth is lost forever. (31.10-14)
This is the problem with secrets…eventually, they die out. Sister Sandrine didn't know that the Priory (or should we say Saunière) had a contingency plan, so she dies thinking that their "precious truth" is lost forever.
Quote #5
"King Godefroi was allegedly the possessor of a powerful secret— a secret that had been in his family since the time of Christ. Fearing his secret might be lost when he died, he founded a secret brotherhood— the Priory of Sion— and charged them with protecting his secret by quietly passing it on from generation to generation. During their years in Jerusalem, the Priory learned of a stash of hidden documents buried beneath the ruins of Herod's temple, which had been built atop the earlier ruins of Solomon's Temple. These documents, they believed, corroborated Godefroi's powerful secret and were so explosive in nature that the Church would stop at nothing to get them." (37.8)
Secrets are often really powerful because by their nature they grant the people who know them a leg-up on those who don't. Even young kids know this fact, and will torment their peers by taunting, "I know something you don't know." No wonder the Priory became such a powerful entity.
Quote #6
In the bizarre underworld of modern Grail seekers, Leonardo da Vinci remained the quest's great enigma. His artwork seemed bursting to tell a secret, and yet whatever it was remained hidden, perhaps beneath a layer of paint, perhaps enciphered in plain view, or perhaps nowhere at all. Maybe Da Vinci's plethora of tantalizing clues was nothing but an empty promise left behind to frustrate the curious and bring a smirk to the face of his knowing Mona Lisa. (40.21)
Some people just have a harder time keeping secrets than others. Poor Leonardo was probably one of those people who just squirmed and fidgeted while trying to keep a bit of juicy gossip from everyone else. That or he just liked tantalizing his fans (he was a prankster, after all).