What leads Caleb to believe that Caitlin is the Chosen One in the novel, Turned (Book #1 in the Vampire Journals)?

Caleb watches Caitlin’s trial before the Assembly, and upon seeing her unharmed in the least by Holy Water, believes she is the chosen One that Doctrine declares will lead them to the sword which will prevent an apocalyptic scenario. As such, he helps her to escape and brings her to the White Coven, which refuses to accept Caleb’s arguments for Caitlin. He is sentenced to fifty years of confinement for abandoning his espionage post, but turns his back on the White Coven in order to stay with Caitlin and protect her.