The Omega Factor by Steve Berry

Enter UNESCO investigator, Nicholas Lee, who works for the United Nations’ Cultural Liaison and Investigative Office (CLIO). Nick’s job is to protect the world’s cultural artifacts—anything and everything from countless lesser-known objects to national treasures.
When Nick travels to Belgium for a visit with a woman from his past, he unwittingly stumbles on the trail of a legendary panel from the Ghent Altarpiece, stolen in 1934 under cover of night and never seen since. Soon Nick is plunged into a bitter conflict, one that has been simmering for nearly two thousand years. On one side is the Maidens of Saint-Michael, les…