"In 2017, Leonardo da Vinci's small oil painting the Salvator Mundi was sold at auction. In the words of its discoverer, the image of Christ as savior of the world is "the rarest thing on the planet." Its $450 million sale price also makes it the world's most expensive painting. For two centuries, art dealers had searched in vain for the Holy Grail of art history: a portrait of Christ as the Salvator Mundi by Leonardo da Vinci. Many similar paintings...