朋友们请看下面背景材料。谢谢！

Langdon's eyes fell to the captain's tie-clip-a silver crucifix with thirteen embedded pieces of black onyx. Langdon found it vaguely surprising. The symbol was known as a crux gemmata—a cross bearing thirteen gems-a Christian ideogram for Christ and His twelve apostles. Somehow Langdon had not expected the captain of the French Police to broadcast his religion so openly. Then again, this was France; Christianity was not a religion here so much as a birthright.