For centuries, believers and skeptics have played tug of war with Shroud of Turin, a 14-foot strip of ivory-colored linen bearing the reddish-brown, bloodstained image of a crucified man. It's the world's most famous relic, revered by millions as the burial cloth of Jesus. 

It's also a lightning rod, sparking thorny questions of science and faith. 

If the Shroud is genuine - 2,000 years old - can science prove its authenticity and miraculous origin (and thus prove the existence of God)? 

If, on the other hand, it's a clever forgery, how does science explain it - an image that's highly detailed yet is so faint and barely-there that doesn't even penetrate the surface of the fibers?

I pondered these questions while researching and writing "The Inquisitor's Key," a crime thriller set in Avignon, France, the home of the Popes for most of the 14th century. 

The book plays the